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91 rough idle, won't rev

damarble

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Need some ideas! 91 Ex with 285k. Mostly original, the prevous (and only) owner said almost nothing has been replaced.

For a while now it's had a rough idle, I cleaned the MAF then replaced it and no help. Now it's gotten so bad it will sometimes die while idling and the truck shakes violently. Also it doesn't want to rev past 3500 rpm and shudders badly if you try to accellerate while cruising.

My first thought is a blocked cat. The entire area around it is extremely hot. I am picking up a vacuum gauge to test it today. The CEL hasn't come on either.

Need this fixed, it's my girls truck and she's got my SHO until I fix it! :(



Quick thread recap of tests:

pulled computer codes, KOEO passed, CM 45, 96
cat = removed, no difference, reinstalled
TPS = 0.89-4.63, within voltage range
MAF = cleaned, within voltage range
fuel filter = replaced, resulted in RPM increase from 3500 to 4400
fuel pump and FPR = assumed good, fuel pressure at 32psi at idle, (30-45 acceptable range)
plugs = not replaced but in great shape
coil/wires = replaced, will rev to redline now but not much power

have not checked = compression (no tester that fits), fuel pressure under load, ACT sensor (where?)
 


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damarble

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The vacuum test wasn't majorly conclusive, then again I've never did it before so I'm not sure. It stayed in the 20 range at idle, dropped when I revved but jumped right back. But at 3500-4000, it dropped to the 15-10 range and stayed. I think that indicates partially blocked cat...
 




FIND

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this may sound like a dumb question, but have you checked your plugs?

May want to check fuel pressure also.

Check TPS as well.
 




damarble

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Well crap. Not the cat.

Maybe the FPR? I'll be checking fuel pressure next.

And no, I haven't checked the plugs. :hammer: Maybe that's next instead....
 




damarble

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I changed the fuel filter and it's a 100% improvement but still not where it should be. I'm suspecting the pump is going out too.

Checking the plugs now.
 




damarble

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The plugs were in great shape, not touching them. I rented a fuel pressure tester and got a reading at idle of 32, which is in the 30-45 range my Chilton's specifies. I think that clears the FPR and fuel pump.

Something is still wrong though, it bogs down at about 4900 rpm now like it has a rev limiter there.

I seafoamed it for good measure but no difference.

TPS now?
 




FIND

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that'd be the next thing to test.
 




1998Explodasport

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I would say you have a bad cat.
 








1998Explodasport

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I would check compression on all cylinders.
 




MrShorty

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When you checked the fuel pressure, did you check the pressure while revving the engine or under load, or was that 32 psi at idle only?
 




damarble

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When you checked the fuel pressure, did you check the pressure while revving the engine or under load, or was that 32 psi at idle only?

I only checked at idle. Didn't even think to check at higher RPMs, just followed the instructions.


Just finished checking the TPS. 0.89v at closed throttle, 4.63 at WOT, within spec. Also rechecked the MAF, within range as well.
 




backfrmcolorado

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check you coil pack, wires, and plugs. my 94 had the same problem and it took me forever to figure the damn thing out.
You can check coil pack by:

1) put on a leather glove (no shock)
2) pull each plug cap on coil pack one by one off, with truck running and hold close to check out how much spark is coming out.

my pack at its best went from 5 cly and i had to dricve to the auto parts store on 3 cyl. Check all that stuffout first then move to fuel.
 




damarble

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Something is still wrong though, it bogs down at about 4900 rpm now like it has a rev limiter there.

I wrote this from memory, forgetting that is the actual redline. I meant to say 4400.


check you coil pack, wires, and plugs

They all seem to get plenty of spark, there was a noticeable difference as I pulled each one. And btw, even my heavy welding gloves did not protect me from shock, got zapped on one of them.




Just tried something I should have done from the start. I pulled the CEL codes even though I never saw a light. KOEO passed but CM threw a

45 - AIR not diverting; Electronic Ignition - coil primary circuit failure

and a

96 - (KOEO) Fuel pump monitor circuit shows no power; (KOER) AIR Bypass (AIRB) not working; (CM) (Service 87 code first if present) Fuel pump relay or battery power feed was open
 




damarble

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I replaced the ignition coil and wires. Slight improvement, it will rev to redline now under load but not too happily.

I assume code 96 - Fuel pump relay or battery power feed was open, is due to me pulling the fuel pump fuse while running to relieve the fuel pressure when I replaced the filter?

I have no clue what "AIR not diverting" means...
 




MrShorty

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I only checked at idle. Didn't even think to check at higher RPMs, just followed the instructions.
I've seen cases where the fuel pump gets "tired" or something else interferes with normal fuel flow. If you get a chance, I'd probably re-check the fuel pressure, and be sure to check the pressure under the same conditions that the engine behaves funny.

I haven't seen a CM 45 on one of these before. And, one of the problems with generic EEC-IV code lists, is you get code definitions that don't always apply. I didn't think Ford even put an AIR system on these, but I could be wrong. One thing I'd probably do would be to get some year/engine specific diagnostic information for this. Most public libraries have professional manuals you can refer to for this kind of information.

Another thing I'd be looking at is the possibility that you are up against some kind of rev limiter. I don't really know if our era trucks have a limiter programmed into the PCM.
 




Vargas

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Not the most fun task, but have you considered cleaning the fuel injectors? A good pump, clean filter, good FPR and acceptable pressure readings don't necessarily mean good delivery. Did you check every spark plug? One poor injector can cause that cylinder to run lean and erode the spark plug. When that cylinder starts misfiring, the unburned fuel causes the cat to run hot. It's a possible scenario, but it should trip the CEL and throw several codes.
 




FIND

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how much power loss are we talking here?
 




damarble

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Thanks for the input MrShorty, I understand you are something of an expert on these computers and I'll follow your advice.

Not the most fun task, but have you considered cleaning the fuel injectors? A good pump, clean filter, good FPR and acceptable pressure readings don't necessarily mean good delivery. Did you check every spark plug? One poor injector can cause that cylinder to run lean and erode the spark plug. When that cylinder starts misfiring, the unburned fuel causes the cat to run hot. It's a possible scenario, but it should trip the CEL and throw several codes.

I haven't thought about cleaning the injectors manually, I just did a Seafoam treatment with no change. I have not checked every plug, just the easiest one to get to, pulling them all for inspection will be on my list.


how much power loss are we talking here?

I'd estimate roughly 30-50% above 3K RPM, under that much more minimal, 20% tops.
 


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damarble

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So dragging this one back, I still haven't got it 100% right. I just swapped in a recently rebuilt 93 motor and it's still not running well. I'm getting a miss on cylinder 5. Before I yanked the motor I was getting misfires too, but I didn't check which cylinders. I used all the sensors and injectors from the 93 motor, but had to delete the EGR crap and swap my wiring harness (auto to manual).

I just replaced the coil and wires have been replaced with known good units. There was an improvement but not enough. I'm going to swap a known good MAF and O2 sensor on for good measure, as well as new plugs (although the ones in the motor were supposed to have been good).
 




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