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Runs Like &$^! Again.


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A few days ago, I drove my truck to the store late at night, and then a few other errands and back home.  ran fine.  The next morning, I go to start it and it nearly dies but goes back to idling fine.  Go to drive it, and wham, like a hammer it won't accelerate at all.  Stumbles, backfires, pops, bucks.  won't go anywhere.

I've already done a ton of work to this thing, and I have even gone back over everything just to make sure something didn't go back and rechecked all of my sensors.  Everything has been cleaned, probed, slapped, kicked, hit with a hammer.  Even checked my new fuel filter to make sure it wasn't clogged. I checked all of the plugs, those are good.
I threw in a new ICM thinking it was a possible candidate, and at first it was promising.  It definitely ran better, but same issue.  I threw the old one back in, just to make sure and yep, it ran worse than it did with the new one. So I put the new one back in.

TPS is re-adjusted and tested.   IAC is new.  Distributor is new and indexed.  MAP checks out fine.  All sensor grounds check out fine.  CPS shows 0.6V while cranking.

Still won't accelerate, and the problem gets worse as it gets up to temp...  Once hot, it barely runs and will die at idle.  I am still going to test the fuel pressure, but the fact that it gets worse as the engine gets to operating temp leads me to think that isn't the problem.  Everything is new except the regulator, and that shows no signs of leaks or failure (and was replaced by PO)

At this point, I can only think it's the ECU...  I am frustrated and at a loss.  Right now I want to fill the bed with wood and gasoline and make a rolling fire pit out of it.  

Is there a way to test the ECU, or should I just gamble with a hundred bucks on a recon one and find out?  Anybody happen to have a Renix ECU laying around not doing anything I could buy?  I would hit up the Junkyards, but a bit dry on renix vehicles at the moment.


EDIT.  Truck is a 1988 Pioneer 4.0 

 

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What's your fuel pressure reading at idle, both with the vacuum line connected to the fuel pressure regulator and with it unplugged? 

I'm testing it tomorrow.  Borrowing a tester from a friend to do said test.  The thing is though, I have had vehicles where the FPR failed, and they never acted like this.  They also never got worse as the engine got up to temp, and then better as the engine cooled again.    It would be nice if it was the regulator though...that's the only part of the fuel system that I personally haven't replaced yet.

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That slightly perplexes me, because the pump is brand new and doesn't even have 500 miles on it.  And currently the tank is completely full, so it has a lot of fuel with which to cool itself.  Also, if that were the case why would it run fine for hours the night before, which was actually warmer than the next morning (cold front came in) and then suddenly not work after running for less than a minute.  It tries to stumble and die almost immediately after being started.  The issue just gets worse as the engine gets up to temp.

I did manage to pick up an ECU from a known running 1987 2dr 5spd cherokee at the junk yard, well it was tagged as such at least.  From what I could tell it looks to have blown a headgasket, or some other type of catastrophic failure judging from the oil spewed all over the passenger side of the engine back below the cylinder head.  I figured for $20 it can't hurt to find out.

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Carter fuel pump, and Yes, the ballast has been bypassed.

Every part I have replaced on said truck has been a brand quality that I do not question.  Most of my sensors are from SMP, (Standard Motor Products) which which I have been very happy with over they years.  I know better than to cheap out on some things.

 

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Ok, so I discovered that the ECU I got from the JY is a Reman unit that is only 2 years old (score!).  Finally got a chance to plug it in and see if it made a difference.  And it did, but it didn't solve everything.

It runs better, way better.  It also now does not die once the engine gets warm.  It actually stays running, and my fluctuating idle is gone completely.  If I rev it up slowly, it doesn't stumble, but does when I rev it fast - It still backfires, but now only through the intake.  You can still tell by the way it's idling that the timing is still not right though.  The only thing left that I can think of is that my harmonic dampener has slipped, and my timing is set wrong and off a tooth at the dizzy or something.  

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Stick a timing light on it and see if the timing advances up to about 14* BTDC as you increase the RPMs and remains fairly steady on that area mark. You can't change it, but looking at the timing can detect a distributor problem.

 

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Yea, I got a timing light in my hands now so I am going to check it tomorrow. The question is, if the harmonic balancer has slipped, then wouldn't it be reading wrong from it's true position?

Pull #1 plug and find TDC then check against HB.

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Well, I was having this exact same problem, checked and cleaned every sensor/connector I could find, swapped known good parts in to test, and back off when they weren't the cure. I kicked myself repeatedly when I figured out what it was. DO NOT OVERLOOK THE SIMPLE THINGS! 

Mine was the brand new spark plug wires I'd put on it at the same time I'd put a new CPS and grounding straps. Put the ancient ones back on and she runs perfectly fine. Wasted a lot of time and money on trying to find some sensor being bad, or checking if the fuel pump was burning out.

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I have another set of wires I could use, but considering they weren't a problem for a couple of months, and then to suddenly be a problem for no reason that I can think of is just strange to me.

Well, yours probably ISN'T the wires, if you haven't changed them for a few months prior, but every last thing you described fit mine to a T and it was my wires, but being brand new, I thought nothing of them, instead my focus was on the new CPS I had to put in due to a lapse in memory while dropping the transmission.

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  • 2 weeks later...

Yea.  It wasn't the plug wires.  At this point, I am thinking it is a problem with my C101 connector.  I am going to go off the deep end and remove that stupid thing from my wiring harness completely.   Although, I still haven't confirmed an issue with potential harmonic dampener slip, but from everything I can tell, my timing is fine.

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Yea.  It wasn't the plug wires.  At this point, I am thinking it is a problem with my C101 connector.  I am going to go off the deep end and remove that stupid thing from my wiring harness completely.   Although, I still haven't confirmed an issue with potential harmonic dampener slip, but from everything I can tell, my timing is fine.

Ditch the C101.

 

Post 27 in my link below. 

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  • 11 months later...

Touching base a year later.  I forgot to update this, so I thought I would fill in the blanks.  
After all of that hair pulling, head scratching, parts swapping, and other such nonsense, I ruled it down to a fuel system issue.  Put a pressure gauge on the rail, and I noticed that the pressure was actually jumping all over the place.  Ruled out the regulator as the problem and decided it had to be the fuel pump, or something in the tank.

So I dropped the tank completely and cleaned it out.  I found a strainer for an old fuel pump in it, I can only assume it came off the stock pump when the last guy replaced the OE unit.  Everything looked normal, but I decided to bench test the hanger and pump with a bucket of fuel from the tank...  Then I discovered the cause....

The rubber line that goes from the fuel pump to the hanger was slipping off the end of the pipe under pressure, and as the pressure dropped it would push back on.  For some reason, this pipe on the hanger has no sort of bulge or flare to keep the hose in place, and I guess I didn't gorilla grip the clamp enough to make it stay in place.  So, I whipped out my 37.5 Degree flare tool, added a hint of flare to the end of the tube and proceeded to king kong that clamp on.  Put it back together, started the truck.  Runs better than it ever has.

I admit, I felt dumb, but I honestly never would have though of that actual problem.  Oh well, here's to free fixes.  On the plus side, I have a spare computer and I got rid of the C101 connector. lol

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Yea, I was pretty ecstatic when it fired up and actually ran like it was supposed to. I even screamed "YEAH!" really loudly, and if I had a buddy with me, we would have high fived.  I honestly wanted to drive it off a cliff once it started that nonsense.  Currently waiting on my drivers floor pan from C2C.  That's my next project.  Then rhino liner, carpet, and headliner, and maybe some sound proofing and new speakers.  Otherwise, I am happy with it.  

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