'89 4.0 L Loses Power Under Load, But Is Ok After Restart

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Hi folks,
I've been browsing this forum since I got my 89 Comanche a few years ago and have picked up many great tips.  Now, I am stuck and request some help.
I have an 89 Comanche with the 4.0L  5speed.  It's a rust bucket and will never go on the road again, but has been good work truck plowing my laneway and hauling my firewood.

  The problem now is that after a few minutes (maybe 5 or 10) of plowing snow, it drastically loses power, and won't respond to the gas pedal. It farts and pops and will barely go at all. It will sit and idle fine, but not rev up. The odd part is that if I turn it off and then right back on again it will work fine for another 5 or 10 minutes, then the same thing happens. I'm not talking about letting it cool down … just off, then immediately on again and it's ok.

 Since I can't take it anywhere, I have asked every mechanic I have ever paid to work on my vehicles for advice. Their consensus is that it is some sort of computer issue since it isn't a consistent problem and seems to reset itself on restart.

I am a handy enough guy but not a mechanic, however I am still willing to try whatever I can do on my own to get it going properly. I don't have any electrical testing tools and even so, I am not sure I could use them properly.

 So my question is…..  should I look at changing ECU?  Most people say it hardly ever goes bad but what else could it be?  I haven't been able to find a used one so I am looking at buying a new (reman) one.

Here's what I have done so far.

I had another coil+ignition control module from a wrecker and tried that (after cleaning all it's connections)
new fuel filter
tighten most intake manifold bolts (some I couldn't reach)
cleaned throttle body
cleaned IAC
reset comp
disconnected and cleaned every connector under the hood I could see
refreshed as many ground wires as I could

checked vacuum lines (I think all of them)

Thanks so much..

PS... I haven't yet cleaned the connectors at the ECU because so far I haven't been able to get it off due to it's awkward spot and my old and inflexible body :-(
 

[HOrnbrod]

Sounds like a possible fuel starvation problem. You really should borrow a set of gauges from Autozone or similar and look at the fuel pressure at the fuel rail port. Also pull your left tail lamp and check / clean / tighten the ground screw under it; it's where your fuel pump is grounded. You can also try bypassing the white porcelain fuel pump ballast resistor on the left fender.

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Thanks for the response.
Maybe I should have mentioned too that the fuel tank has been changed. It's from some other vehicle (not jeep), but the pump and sending unit are still the original… just sort of bent to fit. Not sure if it's relevant.  And the fuel gauge doesn't work of course. 
Yes, while it's happening does feel like gas starvation as you said, but even when it is sitting still not under any load,  I can't rev the gas...  until I restart, and then it's good.  I will double check that ground, but I would imagine that a bad ground would be more inconsistent, no?  
 I can try to get a gauge, but one of the mechanics said " if you get gas squirting out of that port, you should be ok.  The exact pressure doesn't matter for now"  Any thoughts on that? Right now I'm not worried about fine tuning it and mileage isn't an issue; I just need the full power back for plowing.

 

Sorry to be such a newbie but could you explain how to bypass that resistor?  Just join the two wires?   Is it a way of testing or maybe a "fix"?

[Megadan]

Fuel pressure is very important in an EFI system.  System pressure actually determines flow rate at the injector.  If an injector is rated at 19lb/hr at 3 bar of fuel pressure and it loses 10psi, it can now only flow like a 16lb injector.  That difference is enough to generally cause an ECU to max out injector duty cycle because it will detect a lean condition, or simply die from starvation.

IMHO, it sounds like the fuel pump is getting tired.  As it is running it is overheating and losing power.  you shut it off and it gets a chance to cool down, so when you start it again it runs more strongly for a little bit until it starts to get too weak again.  A lot of people say/think that fuel pumps only die instantly, but I have seen many that have given signs similar to yours before they crapped out.

Just my theory on your issue at least.  Not a strong theory though, because your restarts are almost immediate, which doesn't give the pumpa chance to cool much or at all.

[HOrnbrod]

Sorry to be such a newbie but could you explain how to bypass that resistor?  Just join the two wires?   Is it a way of testing or maybe a "fix"?

The ballast resistor is relayed into the fuel pump supply voltage circuit after the engine fires up. It drops the supply voltage to the pump, supposedly making it quieter (pump runs slower) and to make it last longer. Just install a jumper wire across the two resistor terminals, or disconnect the two wires from the resistor and join them together.

 

And yes, low fuel pressure in an MPI fuel injection system can cause low power, stalling, and eventually no power. Just because the pump pisses a little fuel from the test port means nothing. It has to be at the correct pressure and pump a measured quantity of fuel vs. time for the engine to run properly.

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Ok, I will find a way to test the fuel pressure.  It sounds more important than I thought.

 

So, low fuel pressure might cause the ECU to send a false message about the mixture and maybe bypassing the resistor gives more pressure which would help?  Am I understanding the theory?  Any thoughts on why turning it off and on again immediately would make it work ok for another 5 or 10 minutes. Could turning it off reset the ECU and then it would work for a while again?

[Megadan]

A resistor on a fuel pump circuit is designed to limit voltage to the fuel pump.  Fuel pumps are an electric motor, and like all electric motors the more voltage that gets to them, the more strongly they operate, and the more fuel they pump as a result.  In most cars a resistor is generally used on a fuel pump to keep the pump operating at low voltage during low load situations.  Once a set of determined parameters have been met, such as specific RPM, TPS signals, etc, the system will switch and bypass the resistor to get full voltage and more fuel flow.  This is generally done to increase fuel pump life, decrease evaporative emissions by reducing heating of the fuel in a return fuel system, and reduce noise from the pump.  I am not very familiar with how the Renix system operates with it's resistor, and I was never given the chance to learn because mine was already bypassed.  I do know that you can clearly hear my brand new Carter in-tank fuel pump just slightly over my exhaust system, which isn't super loud but not really "quiet" either, from outside of the truck.  I don't hear it inside the cab at all though.

The fact that even when it starts to run poorly when the problem occours, but will still idle ok really indicates a fuel flow problem like hornbrod stated in his first response.  A way to look at fuel pressure when the issue starts occouring will give you a chance to diagnose this possible issue.  With the gauge on the fuel rail, when the issue starts to occour you can rev it by hand and watch what the fuel pressure does when the engine starts to struggle.  If the pressure drops as you rev it up, or doesn't increase as vacuum decreases, then you will have your answer. 

[HOrnbrod]

Intermittent shutdowns are one of the toughest things to diagnose. What we are doing here is eliminating the obvious before jumping on the ECU ($$ for replacement, and they rarely go bad). So test the fuel pressure, if okay, then bypass the ballast resistor. It's windings could be breaking down when it gets hot, so bypass it and run it like that for awhile to see if it helps.

 

Also, it sounds like this truck is rarely driven. How old is the fuel in the tank? Throw a bottle of dry gas in the tank to get rid of any condensation that may be in the gas.

 

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Thanks so much for your help. I had no idea I would get such detailed responses.

   Yes,  the first thing I did was put in some "dry gas." And fresh gas.
It's true that the truck isn't used frequently, but I always add a good fuel stabilizer and try not to overfill it so the gas is fairly fresh anyway. And it's still snowy here so I am using it once a week at least, and more now that I'm trying to sort this out.

I'm all for finding some other cause that doesn't mean buying a new ECU. I'll try to find a pressure gauge to borrow or maybe buy one if it's not too much $.
Umm…. Sorry if this is a really dumb question but… I have a gauge that was on an air compressor … would that work?

[HOrnbrod]

Sorry if this is a really dumb question but… I have a gauge that was on an air compressor … would that work?

A 0-60 or 0-100 PSI gauge will work, but you a hose from the gauge to the fuel rail with a specialized adapter fitting that screws to the fuel rail Schrader valve port. Autozone will lend you a fuel pressure gauge set for free (deposit required) if you don't want to purchase one.

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No such thing as Autozone around here,  but I will see what I can figure out. Maybe just buy one for fun..

I don't understand though how a fuel starvation issue could be quickly but temporarily resolved by simply restarting the engine. That's why I was thinking it was the computer… restarting always helps them!

[HOrnbrod]

In that case, buy a new computer and try it out. I'd eliminate the simple stuff first, but that's your choice mate. Let us know what happens.  :thumbsup:

[Megadan]

In that case, buy a new computer and try it out. I'd eliminate the simple stuff first, but that's your choice mate. Let us know what happens.  :thumbsup:

X2.  Checking the fuel pressure and ruling out the fuel system all together means you can narrow it down.  One of those start simple and work your way up things.  Nothing is more frustrating than spending a ton of money on something to then find out you didn't need it and your probably could have been solved for a few bucks.

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I will for sure check the possible fuel issue first. Even if I end up buying a gauge, it's cheaper than a computer!  I am just trying to understand in my mind why it might be happening.

Thanks again for your advice folks.

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I'm wondering if there is anything I can check while it's not acting up? Will checking the fuel pressure while it's running ok tell me anything ?

 Here's why I ask:
 My local auto parts place can order a gas pressure tester but meanwhile, I decided to go over all the connections again. The one in the taillight was quite rusty, so I just cut it off and regrounded it nicely. I was pretty excited because at first, I couldn't get it to act up and thinking I would report back that I had resolved it, but then last night we had an actual snowfall so I was able to test it for real.  It did go for much longer before it had the problem, but then just as I was finishing up .... I heard the dreaded "put-put-put" sound which I know means it will start missing and lose power soon. Sure enough it did, but not as badly as before.  

 And I don't know whether it means anything, but when it idles after that,  it is actually very low... 300-400 rpm. Its usual idle speed is about 1200 rpm.

 Even after I get a fuel pressure tester it may not snow any more this year, so that's why I'm asking if there is anything else I can check.

 Thanks again

[HOrnbrod]

Do you have a multimeter? If so, check the fuel pump supply voltage while the pump is running at the tank electrical connector by back-probing the hot and ground pins. If you have bypassed the ballast resistor this voltage should be at or near battery voltage if the harness is sound all the way back.

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I haven't yet bypassed the ballast resistor but that's next I guess.

 

I do have a multimeter, but I don't think I will be able to test the connection right at the gas tank because the tank we put on has it's opening and connections on the top rather than the side. Not accessible unless I drop the tank. Once the weather warms up I may do this anyway because there is a leak somewhere at the top too.
Are you suspicious that the fuel pump is failing?

[Aurelio corso]

I had a similar problem and someone installed the fuel filter near the carb in backwards.....

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I didn't know there was a filter up there.  I already changed the one at the back on the frame rail near the fuel tank. Twice.

[HOrnbrod]

I didn't know there was a filter up there.  I already changed the one at the back on the frame rail near the fuel tank. Twice.

 

There isn't. There isn't a carb either.

 

Are you suspicious that the fuel pump is failing?

Yes. But you won't know until you check the voltage and/or the psi reading.

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Are you suspicious that the fuel pump is failing?

Yes. But you won't know until you check the voltage and/or the psi reading.

 

 

Understood. I'll report back when I have more info on those. :)