Ghost Engine Miss

Ok, I'm back to the same problem. What occured was I changed the plugs and wires on my 97' and all the sudden it started missing. I decided to clean the battery terminals to see if that would help. It didn't, in fact it made it worse. When I but the terminals back on my Jeep wouldn't even idle correct and I had to limp it to a mechanic to see what was going on. The mechanic replaced the Flywheel Position Sensor and the Jeep idles fine. But, the first two times I tried to start it, it would only start if the Jeep was in nuetral. That problem went away and now it starts fine, but this engine miss is still here. I am at a complete loss as to why the engine is missing, but I've tried three different sets of wires and two different brands of plugs with no difference. What really throws me off is the fact that the miss seems to be completely random. It seems worse under light acceleraion, when I hit bumps, and when the engine is cold. When the engine is cold it seems to miss alot, almost to the point of stalling. After a while that evens out, but when I accelerate or hit a bump it misses. So, what I'm thinking is that there is either a fuel issue and for some reason it only occurs during these conditions, or there is a loose wire somewhere that moves or grounds out when the engine moves around alot. The fact that it is worse when the engine is cold makes me think there is a sensor that is malfunctioning, but I'm not getting any check engine lights, except for the flywheel position sensor which has been replaced. If anyone has any ideas what this could be, the help would be great. Thanks in advance.

Almost forgot, It also seem to be worse when I am below a 1/4 of a tank of gas. This morning it was really bad, so I stopped to get gas. When the tank was full the problem lessened by alot.

Osprey413 said:

Almost forgot, It also seem to be worse when I am below a 1/4 of a tank of gas. This morning it was really bad, so I stopped to get gas. When the tank was full the problem lessened by alot.

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Have you considered that your fuel pump might be funny? Altering the symptoms by changing the level in the tank seems to me to be a significant piece of information.

I don't recall offhand, but the FSM should mention a fuel pressure spec - you may want to look into putting a pressure gauge on the fuel rail and seeing if it's up to par.

Given what happened when you filled the tank, it may not be missing but fuel starvation that you're experiencing - fuel pump might be weak (odd though on a '97 - my '92 is still on its original at 208,000 miles), or the pickup might be out pf position.

Have you done any damage (dents) to your tank?

Rob

I also agree that it may be fuel related if it gets better when the tank is full.You may also want to check the TPS electrical connections. also pull the distributor cap off and check to see if the rotor is loose or the cap is not sitting on the distributor correctly or is loose. Good luck!

The fuel pump does seem like an unlikely thing to suddenly go out like this, but it could happen. I don't know how to put a fuel pressure guage on my fuel rail, so any help or writeups would be nice. My 87' is still on it same fuel pump with 215000 miles on it, so I dont think that my 97 with less than half the miles would go out. I may take the distibutor off and check or even replace the rotor and cap. Cant hurt i suppose. BTW, every once in a while when the engine misses my RPM's will bounce around. They will drop to about 500 when the engine misses and then when the engine catches it will shoot up to about 4000 RPM and then drop back down the where ever the engine is actually running. Where exactly does the tach get its reading from? Is it the coil or the distributor?

There's a schrader valve on the fuel rail under the hood - looks like a tire valve, I think. You can attach a mechanical gauge there to see what it's doing when the engine is running. Such a gauge should be available at parts shops that rent tools, and a mechanic should have one.

I agree that the idea of a fuel pump failing on that new a rig is odd, but stranger things have happened. It doesn't hurt to do the fuel pressure check - if nothing else, you can then eliminate the pump as a cause.

Rob

It's likely the engine ran "rougher" after cleaning the terminals because you reset the computer when you disconnected the battery. You indicated that you tried replacing the wires and plugs with several iterations of new parts. Are you sure the ignition wires were fully seated in the cap and on the plugs. Did you try replacing with the original parts? If the original parts showed the same problem, consider the other parts you touched -- the distributor cap and coil. Did you check the cap for cracks -- may show as condensation on the inside. Any arcing around the cap? Did you dislodge any coil wiring? Although much of the ignition system is after the computer, did you check for fault codes?

I'm beginning to think the problem may in fact be my Automatic transmition failing. The reason I'm beginning to think this is because when the engine was missing due to the Flywheel Position Sensor going bad, the Jeep didn't buck and jump like it does now. This could also explain why it is worse when the engine/transmition is cold. Perhapse my transmition fluid has gone bad? I smelled the fluid from the dipstick, but it didn't smell burnt or anything. I don't know. I'm about ready to take it to the dealer and let them look at it. My mechanic didn't find anything wrong, but obviously there is something.