K-Jet Debugging Guide

There are a lot of different things that can go wrong with a mechanical fuel injection system. I will try to cover the most common symptoms and their usual suspects. I have included some basic info for testing things common to all cars (like the throttle body sticking) but this mainly covers K-Jet related problems. This is not intended to be a full course in fixing cars in general, so I’m not going to cover how to check your spark plugs or distributor cap and rotor or reset your timing belt.

Note: READ THE CI GREENBOOKS. Also, please refer to the K-Jet in Detail article for diagrams and descriptions of the K-Jet components.

Note: If you have not replaced your engine wiring harness (1980 to 1987 cars) replace it first. It can cause a lot of problems on its own. Due to the random nature of wire breakdown, the problems associated with a bad wiring harness can take almost any form. If you want to do it on the cheap, read this article.

Difficulty starting engine warm or cold

1) Vacuum leaks
2) Other air leaks
3) Airflow sensor plate out of adjustment or sticking
4) Fuel distributor clogged
5) Low fuel pressure (line pressure)
. a) bad fuses or fuse block
. b) bad fuel pump relay
. c) bad fuel pump check valve
. d) clogged fuel filter
. e) bad fuel pump
. f) out of gas
. g) line pressure regulator out of adjustment

Difficulty Starting Engine Only When Cold

1) Control pressure too high
2) Auxiliary air valve or idle air motor not working or sticking
3) Problem with cold start injector / thermal time switch circuit

Difficulty Starting Engine Only When Warm

1) Cold start injector stuck open or leaking
2) Control pressure too high or too low
3) Regular fuel injectors leaking
4) Wrong rest pressure

Idle fluctuates or ‘hunts,’ surging in gears 1-4, abnormally high idle

1) Idle air motor not working properly
2) Throttle plate improperly adjusted
3) Bad wiring to CIS Computer
4) Vacuum leaks

Random no-start, or will not start unless left in Key Position II for a few minutes

1) Wiring problem at ballast resistor
2) Wiring problem at starter or coil

Difficulty starting engine warm or cold

If you turn the key and NOTHING happens, you probably have a dead battery or the wires that connect to the starter solenoid are disconnected. However, if you turn the key and the starter motor engages and the engine turns over but just won’t start, here are some common causes of this:

1) Vacuum leaks: The K-Jet system is very sensitive to vacuum leaks. It is dependent on vacuum lines due to the mechanical nature of the system. Unlike some EFI systems, K-Jet cannot compensate for vacuum leaks. If there are air leaks in the system, more air is getting to the engine than is being measured by the air flow sensor plate. Because of this the air/fuel mixture is lean meaning that there is too much air in the mixture. Vacuum leaks can also cause abnormally high idle problems, as well as many other weird symptoms.

The easiest way to check for vacuum leaks is to look. Check out your vacuum hoses. Are they all connected? Are any cracked or broken? Are they hooked up like the diagram under the hood shows (Vacuum hose routing sticker)?

Vacuum hose is cheap so it’s a good idea to just replace all the lines when you get your Volvo. Major vacuum lines, like those coming from the idle air motor, the auxiliary air valve, or the brake booster, will make a loud POP! the first time they come off, and after that the car won’t start at all.

Tips for preventing vacuum leaks:

  • Always keep your vacuum lines in good condition.
  • Use the right size of vacuum lines (metric!).
  • When you replace them, use small hose clamps or cable ties to make sure that they are tight and don’t pop off.
  • Use quality (OEM Volvo) gaskets (intake manifold, throttle body).

2) Other air leaks: If you can actually get the car running, but it runs rough or has trouble starting, you can check for air leaks at the intake manifold gasket (where the intake manifold attaches to the head) or at the throttle body gasket (where the throttle body attaches to the intake manifold) or at the injector seals (where the fuel injectors are inserted into the engine.)

Check for air leaks at these places by spraying a flammable solvent (such as WD-40 or carb cleaner) around the gasket or seals. If the idle of the car speeds up, it means the engine just sucked in that flammable solvent through an air leak.

To fix these problems, you probably need a new gasket or new injector O-Rings. These are also inexpensive parts, even from the Volvo dealer.

3) Airflow sensor plate out of adjustment or sticking:
This is a very unlikely scenario, but it can happen. Basically, you have to take your fuel distributor and air flow sensor apart and re-calibrate it. Like I said, very unlikely that this would happen.

4) Fuel distributor clogged:
Again, this is an unlikely scenario, but it can happen. Mostly it is caused by some contaminant in the fuel caused by a low quality fuel filter or by someone putting sugar in your gas tank. Anyway, fixing this requires dismantling the fuel distributor and cleaning it all out with carb cleaner or the like.

5) Low fuel pressure (line pressure):
Low line pressure means that not enough fuel is making it to your fuel distributor. This could be caused by many different things, which I will go over in detail.

a) bad fuses
or fuse block
Believe it or not, bad fuses account for a large number of K-Jet no-start conditions. The main fuel pump and the in-tank fuel pump are fused and if one of these two fuses is blown or poorly connected, your fuel pump won’t work. The best way to remedy this is preventative maintenance. Your fuse panel is located behind a plastic panel to the left of the driver’s left leg, below the steering wheel and in front of the driver’s seat.

With the battery disconnected (so you don’t kill yourself) remove all the fuses in the fuse block and scrub all of the terminals with a wire brush. You can get a wire brush about the size of a toothbrush that is perfect for this at any hardware store for about $1. Once all the terminals are clean, put the fuses back in with the right amperage ratings in the right spots. Replace any fuses that are corroded or burnt.

b) bad fuel pump relay

The fuel pump relay is a very common cause of failure to start or of your car dying when you are driving it. A relay is a fairly simple device that allows a low amperage electric circuit to turn on a high amperage circuit. This way, you don’t have to run a high-amp circuit for a very long distance, reducing the risk of showers of sparks and fire etc. The fuel pump relay does just this – it takes a signal from various components in the car and turns on the fuel pump when it’s supposed to go on.

If you can’t hear your main fuel pump buzzing (running) when you try to start the car, suspect the relay before you suspect the pump. The fuel pump is located underneath the car on the drivers side.

The fuel pump relay on a K-Jet car is located underneath the steering wheel and behind the black felt mat that protects the underside of the dash. It has six spade connectors on it and is rectangular; the size is about about 1 inch by 2 in. by 2 in. It can be green, red, yellow or black in color. You’ll know it is the fuel pump relay because it has the following wire colors: red, red/blue, blue, red/white, black, and yellow/red (at least on most cars. Pre-1981 cars may be different.)

The fuel pump relay is cheap, like $25 or so from fcpgroton. Some people take out their old relays and try to re-solder the connections on them, but hey, it’s $25 freakin dollars that might save you from getting stabbed because your car broke down in a bad part of town.

c) bad fuel pump check valve

The fuel pump check valve is attached to the fuel pump. It keeps fuel from running back down the fuel line and back into the tank. It is a cheap ($5) fix, but rarely breaks, since it is just a simple one-way valve. It is silver and is attached to the outlet side (opposite from the gas-tank side) of the fuel pump. If you have a bad check valve, the car will usually still start, but it will be harder to start (takes a few tries every time or the fuel pump will be noisy because it is working too hard, or both.

d) clogged fuel filter

The fuel filter is located under the hood, on the firewall, on the driver’s side of the car. It is about the size of a 12oz soda can. Earlier 240’s (not sure what year delineates this) had the fuel filter located on the firewall as well, but more toward the center. Buy an OEM (Bosch) fuel filter or better! I cannot stress this enough! The fuel lines in the fuel distributor are small and you can save yourself a lot of time and money by having a good fuel filter. Where does gas come from? The middle east! What do they have a lot of over there besides oil? SAND! Does gas have sand in it? You bet! What keeps the sand from clogging up your fuel distributor and causing you lots of problems? Your fuel filter.

The easiest way to test your fuel filter is to unhook the passenger’s side of it and see if any fuel is making it through the thing. If no fuel comes out, the your fuel filter is clogged up or you have a fuel delivery problem (bad fuel pump, check valve, or fuel pump relay.) If just a little fuel comes out (like a tablespoon or so) it is probably clogged as well. If a couple cups of fuel comes out, your fuel filter is probably fine.

e) bad fuel pump

Fuel pumps are expensive ($100+) but they do die occasionally. The easiest way to do this is to start your car and listen for the fuel pump running. If you can hear it buzzing, it’s working. If you can’t check your fuel pump relay first. If the relay is new, and the fuel pump still doesn’t run, you probably need a new fuel pump. The wiring to the fuel pump could be bad (rarely), so you can inspect it first. You’ll have to remove your back seat to find the wires though. You can also definitively check the pump by hooking it up to a battery or booster pack. Yellow is positive and black is negative on the fuel pump.

f) out of gas

Just in case you forgot to check. I’ve done it before.

g) line pressure regulator out of adjustment

The line pressure regulator is a component of the fuel distributor. It can be adjusted by shimming it with these little metal discs. This is kind of a complex process and you need to have a fuel pressure gauge to tackle this. (I am eventually going to write up an article on the fun things you can do with a fuel pressure gauge but for now I am not really going to cover these kinds of things.) In any event, it’s pretty unlikely that this would happen anyway.

Difficulty starting engine only when cold

This is one of the most common problems with the K-Jet system. What happens is you turn the key and the starter motor engages and the engine turns over, but the engine just won’t fire up.

Here are some common causes of difficult cold-starting:

1) Control pressure too high (Control Pressure Regulator) The control pressure changes as the engine warms up. The pressure is created by the Control Pressure Regulator (CPR). The CPR has a lower pressure on the control plunger in the fuel distributor when the engine is cold compared to when the engine is warm. The lower pressure allows the control plunger to let more fuel into the engine when the engine is cold. If the control pressure is too high, the air/fuel mixture will not have enough fuel, and thus the engine will not start.

The common symptom with a bad control pressure regulator is that the engine takes about 20 tries to start, each time idling for a little longer after it starts. Basically, you turn the key, the rpms go up to about 1500 – 2000 when the cold start injector fires, and then the car dies because it is not getting enough fuel after the cold start injector shuts off. Also, a bad control pressure regulator can make the engine almost die when you shift in to neutral (auto trans.) or depress the clutch pedal (manual trans.)

Fixing this often requires a new control pressure regulator (a fairly expensive part). However, many people have achieved success by cleaning out their control pressure regulators. The come apart fairly easily. First though, the CPR must be removed from the car, which usually requires removal of the intake manifold first. However, I have replaced the CPR on a N/A 240 without removing the intake manifold. I removed the distributor cap, the intake manifold support bracket, and the rubber bellows, and I had enough room to move. for turbo models, this probably could not be done.

The CPR is held to the engine by two hex bolts, and is held together by four flat-head-screwdriver bolts. Be careful! The CPR contains several springs and other fragile components. Take it apart indoors in a clean environment where you can keep track of all the little pieces. When it comes apart, there are three metal pieces held together by two gaskets in a sort of sandwich. If your gaskets are ruined (hopefully not) you can replace them with that gasket maker stuff.

Once you have it apart into these pieces, you’ll need to remove the 10mm nut on the heating element and carefully bend the heating element out of the way by bending the two wires that attach to it. Now, use a small flathead screwdriver and remove the four bolts that hold down a circular diaphragm. Be careful!. Once you have that taken apart, get out your carb cleaner and spray out the insides of the thing as best you can. Now, put it back together, making sure that everything goes back the same way it did before.

2) Auxiliary air valve or idle air motor not working or sticking: depending on which one of these devices you have, the symptoms may be different. I’ve never had a bad auxiliary air valve myself, but if I did, I would probably have just replaced it with a “new” one from a junkyard car. If the auxiliary air valve sticks in the open position, the car will idle very high, about 2500 rpms. If it sticks in the closed position, the car won’t idle at all, but may run if you keep your foot on the accelerator to let air into the engine.

A sticking idle air motor will often cause a “surging” when driving – your car will accelerate when it’s not supposed to – and/or a “hunting” idle, where your car just can’t seem to find the right idle. A hunting idle might range from 500 to 2000 rpms, and just keep going up and down.

An idle air motor that just plain won’t work can be detected by trying to start the car and having another person “feel” the idle air motor. It vibrates (buzzes) when it is running. You can also hook it up to a battery or booster pack to test it. Supposedly the middle terminal on the plug is positive and the two outside terminals are negative, but I have been unable to verify this other than just hearsay. An idle air motor that is REALLY dead will actually emit white smoke when you try to use it.

The idle air motor can be cleaned out by removing it from the car completely and spraying it out with carb cleaner. Wear gloves and goggles! This stuff can burn the bejeezus out of your skin and eyes! Spray a bunch of carb cleaner in it and plug both of the ends. Now, shake it up as much as you can – up and down and roll it between your hands like you were trying to make a long snake out of Play-doh like in kindergarten. When you roll it, you should be able to hear it clicking back and forth. If you can’t hear anything moving in there, keep cleaning it or get a new motor. Make sure to shake out all of the excess carb cleaner and let the thing dry for a bit before you re-install it.

3) Problem with cold start injector / thermal time switch circuit The cold start injector provides a brief blast of fuel only when the starter motor is running (when the key is in position III). It operates based on info from a thermal time switch, which tells the cold start injector when the engine is cold. If the engine is warm, the thermal time switch breaks the circuit and the cold start injector doesn’t fire.

Testing the cold start injector is fairly easy. Just remove it from the intake manifold (and make sure to bolt down the ground wires again). Now have a friend start try to start the car while you hold the cold start injector in a small bottle or napkin or some such. If it sprays out fuel, you’re good to go. If it doesn’t, then you’ve got a problem.

The best way to try and solve this problem of whether it’s the cold start injector or the thermal time switch causing the problem is to get out your voltage meter and hook it up across the two little plugs in the plug that hooks up to the cold start injector. If, when you start the car, you get 12 volts across the cold start injector plug, then the thermal time switch is working and the cold start injector is faulty. If you don’t get 12 volts across the plug, then the thermal time switch needs to be replaced.

Difficulty starting engine only when warm

1) Cold start injector stuck open or leaking: basically, this test is the same as the test above for the cold start injector, but you must have the engine warm before you test it. If the cold start injector fires when the engine is at normal operating temperature, then you’ve got a problem. You can do the test for 12 volts across the injector plug to see if it’s the injector or the thermal time switch causing the problem.

2) Control pressure too high or too low: If you are blasting unburnt fuel (black smoke) out of your tail pipe, then your control pressure regulator is giving too low of a control pressure when the engine is warm and needs to be cleaned or replaced as described above. If the control pressure is too high (less likely) then the fuel mixture will be too lean (too much air, not enough fuel).

3) Regular fuel injectors leaking: Again, this would probably be signified by unburnt fuel coming out of your tailpipe. If the injectors leak, then a bunch of gas flows into the combustion chamber when the car is sitting. After the car has been warmed up and has been running, turn off the car and pull out your fuel injectors and see if any are leaking. Be careful, the engine is hot of course. If they leak more than one drop every 15 seconds, they should be replaced. If all of the injectors are leaking by about the same amount, the rest pressure is probably too high, so see below.

4) Wrong rest pressure: The rest pressure is the pressure that the fuel system stays at when the car is not running. If it is too high (pretty unlikely), fuel will be leaking out of all four injectors. If it is too low, the car will have a hard time starting because there is not enough pressure at the injectors, but may eventually start. The rest pressure can be adjusted by adjusting the line pressure regulator, which is a component of the fuel distributor. As I said before, I am not going to cover the line pressure regulator in detail because you need a fuel pressure gauge to really do it right.

Idle fluctuates or ‘hunts’, surging in gears 1-4, abnormally high idle


1) Idle air motor not working properly:
the idle air motor is only found on cars with CIS, and a description of its duties can be found here. If the idle air motor is clogged up with gunk, it can’t change the idle speed fast enough, and the idle will often hunt up and down, eventually finding the right idle speed after several times up and down, ranging from say 300 to 2000 rpms. Often, putting in the clutch or shifting in to neutral will make the car almost die, because the idle air motor cannot catch up quickly enough to balance out the idle. If the motor is really shot, it will emit white smoke when it burns out. You won’t be able to see this smoke unless one of the idle air hoses is disconnected.

Another way to test the idle air motor is to measure the resistance with your multimeter. A good motor should read +/-20 ohms resistance between any two pins that are next to eachother and +/-40 ohms resistance between the two outside pins.

The idle air motor can be cleaned out if it is not too damaged. The most common way to do this is to detach the motor from the car and spray it out with carb cleaner. Fill the motor with carb cleaner and plug both openings in it. Shake it up and roll it back and forth in between your hands like you are making a ‘snake’ out of clay. You should hear it clicking back and forth. Repeat the cleaning process several times, drain the motor, and then allow it to dry for an hour or so.

Surging is when the engine RPM’s jump up unexpectedly while driving, as though a ghost had stomped on the gas pedal. This is almost always caused by a bad or sticky idle air motor. If the surging only occurs in Overdrive (manual transmissions), then it is probably being caused by a short or bad wiring to the overdrive solenoid.

2) Throttle plate improperly adjusted: will force the idle air motor to work too hard (if the throttle plate is too tightly closed), and may even require it to work beyond its capacity, creating the same symptoms of a failing idle air motor. The throttle plate is inside the throttle body and can be adjusted by turning a small screw near the throttle cable spool with a screwdriver. The throttle plate should always be slightly open to allow some air to go by, otherwise all of the air that the engine burns at idle will have to travel through the idle air motor.

Also, make sure that your throttle microswitch is properly adjusted when you check the throttle plate. For example, if the microswitch is not “clicking” when you open and close the throttle, you need to adjust it or replace it. Also make sure your throttle cables are good and tight, but not so tight that they are holding the throttle body open on their own.

3) Bad wiring to CIS computer: when replacing the engine wiring harness, many people do not replace the wires that run through the fire wall to the CIS Computer. These wires do not come with a new wiring harness and must be replaced by hand. They often need to be replaced, especially on high mileage cars, due to their close proximity to the exhaust manifold. Inspect the wire sheathing by unwrapping the electrical tape that covers this wire loom on both sides of the firewall.

Another possibility is that the wiring or wiring terminals are corroded. I recently had a surging idle problem on my 1985 245 Turbo that would happen so randomly it was difficult to diagnose properly. I took the wiring off of the CIS computer to test a different computer (which could have caused it, although it is extremely unlikely) and I noticed that several of the terminals were corroded. After cleaning them with a wire brush, everything works fine again.

Random no-start, or will not start unless left in Key Position II for a few minutes

Note: This is not a problem caused by K-Jet, but is common enough that I decided to include it.

1) Wiring problem at ballast resistor: This problem came to my attention because it happened to a friend of mine. The cause of this problem is usually a wiring problem at the ballast resistor. This can also cause a random no-start scenario. Corrosion, overheating, or just plain old wiring can make it difficult for wires to conduct electricity. On K-Jet cars with the Bosch ignition system, there is a ballast resistor near the passenger’s side firewall, as shown in this picture as part G.

The brown wire that comes off of the starter and goes to the ballast resistor. There is also a blue wire that connects to the other side of the ballast resistor. If the brown wire is not providing enough voltage to the starter, the car will not start unless the ignition is left in Key Position II for a few minutes. This is usually caused by corrosion on the contacts at the ballast resistor.

Cars with Chrysler (white-cap) ignition can have this same problem (I know from experience) even though they lack a ballast resistor. These cars still have the connections in the same spot, and corrosion at this point can cause the car to randomly fail to start.

2) Wiring problem at starter or coil: Same as above, but the problem is at the starter or coil. Check for disconnected wires or corrosion. This is a much less common cause of a no start condition when compared to the ballast resistor.

Also, check the wires between your ignition module and distributor, and between the module and the coil. One time my car decided to suck the red/white wire from the coil into the fan and chop it up, and the car died on the freeway.