Engine will sometimes not rev
 

So I got this annoying problem, it does't happen often but it's annoying. Sometimes when I start the car (91 FC) it doesn't want to rev past 2k :dunno:, just bounces off the 2k mark. It isn't temp related because it happened with cold and engine and warmed up engine. Any help would be appreciated.

how is the idle when this happens? does it change? idle lower, lumpier etc?

I didn't pay much attention to idle when it happened, but I believe it was normal since the engine didn't want to stall

Any Check Engine Light?

If so, what are the codes?

If no CEL, when was the last time you changed the fuel filter?

The check engine light is never on, and even if I check codes by grounding the test connector it doesn't say anything. I didn't get a chance to change the filter yet since I had the car for not even 2 weeks. If the filter was bad wouldn't the problem occur every time, or it gets clogged and then unclogged by itself?

Does your CEL come on for a few seconds with all the other idiot lights during the self-test before you start up? If not, the bulb might be bad or missing - suggest checking that first, fix as necessary and re-checking the error codes.

The CEL does come one during self-test, than disappears and never blinks to throw a code

Does shutting the car off and the restarting fix the problem? If so, maybe ECU related (NO CEL, but still in limp mode). If not, how does it hit 2,000 rpm? Does it rev fast to 2000 and then just bounce, or does it rev slowly? Does throttle position have any bearing on it?

Shutting the car off is the way I always "fix" it. It does rev slow and rough/bumpy till it bounces at 2k. If I give it throttle where it would normally rev to 5k it still bounces off.

I have to think that it is an ECU limp mode thing. I don't understand why the CEL is not getting set. It may be something with the position feedback from the OMP (that is the usual culprit).

Try unplugging the IAT sensor while the engine is running and see if the CEL comes on. I am pretty sure that it should.

I will try it, also want to drive around wait for it to happen and do the self-test while its happening since every time it happens I'm not close to my house and don't have a spare wire.

Still didn't get a chance to check the codes, hopefully my car will break down soon :cuss:

LOL try unplugging the AFM plug to see if the stays running with it unplugged. And your engine light should come on.

Wouldn't that disable his fuel pump?

That is why I always do the Intake Air temp sensor, it is easy to unplug and doesn't cause any major harm to run without it. (You have to have a warm engine).

You can unplug the TPS too (I just tried it, it will throw a code too).

I'm thinking the OP is having wiring harness issues. Corrosion and heat exposure after 20+ years can cause intermittent opens/shorts in your wiring, causing all kinds of problems.

Finally got some work done on the FC like, new oil and a Mazda oil filter, but more importantly about the problem. I unplugged the IAT and the CEL went on, so that works, turned the car off and did the self-test, got a long blink and a short blink so that's 11 to my understanding, which would be IAT. Now all I need is to drive the car and break down so I can do the test then.

You might be right, I do think it is the wiring, I also have a wire issue with the warning cluster, it keeps resetting and sometimes just stays off.

The warning cluster problem (i.e., clock resets itself to 1:00 or stays off) is most likely just the cold solder joint issues common to most FC electronic accessories - no relation to your engine problem, but it be could be a factor in why the CEL isn't always displaying errors.

Simple fix involves removing the cluster and resoldering the larger solder connections between the circuit board and the one connector plug. I did this when I first bought my '7 (over 10 years ago) and haven't seen that problem since. There's was a good "how to" article for this procedure with pictures on the web, but the link I had to it on my PC now returns a "page not found".

When I check the CEL codes the cluster is on, also thank you for advice I shall fix it asap

Well, the car finally broke down, didn't throw any codes and CEL didn't come on. Idles rough around 600rpm. It seems to happen when it's warm outside. Can it be TPS related?

It could be an intermittent vacuum leak for the rough idle, but if it is coupled with the inability to rev, it almost has to be limp mode.

This is puzzling.

I tried unplugging the TPS on my '91 and revving the engine and it revs just fine. I also tried without the MAP sensor - same results. It almost has to be an Oil Metering Pump feedback issue (I don't remember what all codes can set the limp home mode).

Did shutting the key off and turning it back on fix the issue this time too?

I wonder if the CEL is not setting because the fault is immediately clearing, but the limp mode is latched on.

Key method seemed to fix it again. Is there any way to check if its the OMP while the problem is happening? Check wires for resistance and continuity sort of thing.

Best bet is try unplugging the OMP and see if you get the same behavior consistantly (don't let it run too long with the pump unplugged) You could try cleaning up all the connection points between the ECU and the OMP.

I think that if I remember correctly, there is a motor in there to adjust the position and a potentiometer to indicate position back to the ECU (closed loop control) If the resistance does not react as expected, you can get the code. You could have corrosion on the wipe contacts of the potentiometer too (a 'dead spot').

I don't know how to tell you to test that.

You could always try a different OMP, I have one, but it hasn't been on a running engine in years.

My friend has an OMP I can use, he has been premixing for a year now. Guess I'll try unplugging the OMP for a short time, It wont take long to notice the problem. If same things happens I'll check wiring and if that doesn't help ill change out the pump. Should get back with the results this week.

Talked to a good a friend of mine, he believed it is not the OMP because it would throw a code and it will only cause limp mode after a while, mine starts right away. We think it is the MAF, since he had similar problem and his MAF was the source, he also didnt have any codes when the MAF went bad. Time to check MAF wiring.

Not a bad idea, keep us posted!

Your friend is wrong regarding the OMP. When my old wiring harness was going flaky, I discovered that even intermittent open circuit associated with the OMP wiring, either the OMP stepper motor or the OMP position feedback sensor, would both trigger the CEL and force the limp-home mode every time.

Looked and my MAF plug today, wires where under pretty high stress going into the plug, fixed that, cleaned both sides of the connector, need to buy Volt Meter thing sometime soon so I can check the wires internally.

He might have been wrong, but I doubt it. And I don't get CEL, so I don't think its the OMP.