missing after spirited driving

[nonamekid]

This problem has been bugging me for awhile. A little background first so you understand what I am working with. When I bought this 1991 festiva the motor was not in it and I was told the motor was dumping oil out all over the place. So I put the motor back in the car and started it up. I found that the camshaft seal had come out. I put a new one in and that fixed the major leaking problem. I have been driving this car for a couple years and have taken it on road trips. I have taken it on a one day 750 mile road trip with minimal problems. The engine goes through about 1 quart of oil every 2-3 gas fill ups. It leaks and it smokes.

Here is where the problem is. When I stomp on the gas and get the car up to about 4000-5000 rpm and then shift into the next gear, it will start missing. It will miss for about 3-5 minutes after that and then the miss will go away. If I drive the car nicely, it gives me no problems, it is only when I do spirited driving. I have changed spark plugs and the condition still exists. I figure the motor is due for replacement but I was hoping to hold off on that till I get some other projects out of the shop.

[mechanicaldj]

a weak coil, ignition module or bad injector?

just some of what could be

[JohnGunn]

Here are a couple of observations. There is a switch in the throttle position sensor which signals when the throttle is over 70* open. The ECU responds by increasing fuel supply. Since the problem seems to appear when you are triggering this signal, you might want to check the adjustment of the TPS to make sure its signals are being sent at the proper times.

Also, when you reach 4,700 rpm or more the ECU increases fuel injection. It might be helpful to try and zero in on exactly what is the lowest rpm that will cause the problem to present itself. You say between 4000 and 5000, so try going only up to 4000 and see if it happens there, then on up by 100 rpm steps until it first starts to miss.

Also it would help me if you could give a clearer picture of what you are experiencing. For example, you talk about going from one gear to another. Give me the shift where you notice it the most, or is it exactly the same no matter what shift you are making as long as the first gear is able to reach the required rpm?

Then you say the engine will miss for 3-5 minutes after that shift. Is that number always consistent no matter what you do. Could you immediately slow down and still feel the missing or does it happen only if you continue your aggressive acceleration? If you were to immediately pull of the road and allow the car to idle would it miss?

This is an interesting problem. The one good thing about it is that it fails reliably and attempted fixes can be quickly evaluated.

John Gunn
Coronado, CA

[nonamekid]

Here are a couple of observations. There is a switch in the throttle position sensor which signals when the throttle is over 70* open. The ECU responds by increasing fuel supply. Since the problem seems to appear when you are triggering this signal, you might want to check the adjustment of the TPS to make sure its signals are being sent at the proper times.

Then you say the engine will miss for 3-5 minutes after that shift. Is that number always consistent no matter what you do. Could you immediately slow down and still feel the missing or does it happen only if you continue your aggressive acceleration? If you were to immediately pull of the road and allow the car to idle would it miss?

This is an interesting problem. The one good thing about it is that it fails reliably and attempted fixes can be quickly evaluated.

John Gunn
Coronado, CA

I would rule out TPS, because I can floor it in the low rpms and it won't give me problems.

I can get it to happen in the shift from 1st to 2nd, 2nd to 3rd, or 3rd to 4th. After I do this I typically am at the speed I need to be, so I go to the highest gear I typically would use, and it is missing. It does not want to idle, I have to give it gas or it will die. If I push in the clutch, it will die.

And when I say miss, it is a completely dead cylinder, not a weak cylinder. If you go pull a spark plug wire off, it will be exactly what I experience.

The service engine light came on the last time it happened, but the light went away when the miss went away. Will the code still be there if I try to retrieve it?

One other thing, on some days, rainy days most of the time, it will miss in the low rpms if I press the pedal down too much. If I back the pedal off just a little, it will stop missing. If I get the rpms up, it requires me to press on the gas more to get it to start missing.

I have another festiva that is running fine, so I will try swapping parts and see if that fixes it, but I didn't know where to start.

[JohnGunn]

I've been thinking about your problem and believe I've come up with a better explanation for what is going on. It may be related to the effect of heat that is built up as more and more current flows through the electronic parts at higher rpm.

It's like the CPU of a computer. The temperature rises in direct proportion to the load that is being placed on it.

Already you notice that two of the electronic components of the ignition system have built in heat sinks, the Ignition Module and the capacitor, both located close to the coil. I would imagine the Crankshaft Position Sensor, located in the distributor, would fall into that category as well. I can also see how heat build up in the coil might lead to mechanical expansion which could temporarily short some of the windings.

To isolate the problem you might do the following:
1. Drive in a manner to reproduce the problem.
2. Shift into neutral and keep the rpm up while braking to the side of the road.
3. Quickly get out and touch all the ignition parts you can. Ignition Module, capacitor, and coil.

If you weren't physically separated from those parts when the problem was presenting itself, I would suggest you immediately spray the parts with brake kleen to see if that corrected the problem. If you had someone to help you, he could drive as above and while he kept the rpm up you could jump out and spray the parts and observe the response of the engine each time you sprayed a part. Since you have a source of replacement parts, that might be an easier approach.

One other possibility would be to try and take better advantage of the heat sinks on the Ignition Module and capacitor by:
1. Removing the part. Using fine sand paper, 1000 or 1200 grit, smooth the metal surface of the metal heat sink and the metal surface they touch when installed.
2. Apply some thermal grease to the surface of the heat sink, install and test.

Thermal grease comes in two forms. One for applications in which the two metal surfaces are forcefully pressed together by some clamping mechanism. And an adhesive form for applications which do not have this advantage. In looking at the way they are attached, I don't think the designers thought there was need to maximize the cooling effect of the heat sinks. To try and extend the life of an Ignition Module it might be worth using the adhesive thermal grease, for better heat transfer.

The service engine light came on the last time it happened, but the light went away when the miss went away. Will the code still be there if I try to retrieve it?

I've never had to check for an error code, but from what I have read, the process is quick and painless. Ground the right connector. Turn on the key and look for a blinking engine check light. If you do this and it doesn't blink, I would think the computer doesn't have anything to say. But, somehow, I would think it might just remember the last code until the computer has been reset. It's certainly worth a try. It might actually tell you exactly what part is causing your problem.

John Gunn
Coronado, CA

[JohnGunn]

Today I looked at the Ignition Module and capacitor on my '90 L. I was wrong when I said the heat sinks were not in good contact with the underlying metal. Still might be worth removing them to see if they have thermal grease. Even if they do, replacing it with a fresh application should improve the heat transfer. Clean with brake kleen before applying the thermal grease.

John Gunn
Coronado, CA