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88 Barely Running, No Power, Popping, Backfiring...can't Figure It Out...


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Hey folks,

 

I was out wheeling the other weekend and my 88 died on me.  It was running just fine until I stopped for lunch.  It started up and was running pretty good until I felt a bit of a stumble.  I had to make a quick stop to unlock my hubs, and when I hopped back in and tried to start... it wouldn't!! 
I can kind of get it started but I have to be pumping the peddle like made.  and when it does start, I have to keep pumping the peddle and keep the RPM's up around 3000!!  And if I try to drive it, it has zero power.  Any RPM below that and it will just die.   While it is running, its popping, bucking, backfiring and smelling very very very rich.

I kind of figured it might be the ICM and/or coil, so I replaced it from my parts truck that I know runs...  no luck, same issues.  I can hear the fuel pump priming, and I have a good squirt of fuel from the schrader valve on the fuel rail, so that should be good.  I can't think of any other sensor that would cause something like this.  My O2 sensor is new, but if it did fail, it would just go into a limp mode.  The TPS is sending voltage and is adjusted correctly.  The CPS seems to be working, but usually when it dies it just dies, and doesn;t slowly die.  Every other sensor I can't see it causing this problem.  The plugs are new, wires in good condition, and the cap and rotor are ok ( I sanded the contacts which made no difference).

 

ANy other thoughts or ides?

Thanks

Ryan

 

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Sure it's not the fuel pump?  Possible you sucked some junk off the bottom of the tank when you were wheeling.  I'd go to your local parts store and rent a fuel pressure gauge and test to see what your pressure is on the fuel rail.  Should be ~31-32 psi with the vacuum hose connected to the FPR @ idle and ~40 psi with the vac hose disconnected @ idle.

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Thanks for the replies...

 

Because the truck won't idle, I'm unable to check the fuel pressure like you described.  Best I could do is check the pressure the pump puts out when primed.  And when pushing the shrader valve, it looks very similar to what it was like before this issue.  I'm not saying that it might not be it, but it is still putting out some descent pressure.  I will try and get a pressure tester though and see what it is when primed.

The hose to the MAP sensor looks good and the connections look good as well.  I'll try swapping the sesnor from my parts truck and see if that could be it.  But if I recall, I was seeing a pressure reading when I hooked up my Snap-on MT2500 Scan tool.  Too bad this system doesn't give codes!!

Its not a plugged cat, because I don't have one..

 

Thanks

Ryan

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Spritzing fuel at the rail Schrader valve means nothing except the pump runs and creates some pressure. For the HO models there is a simple test that eliminates fuel system physical problems, including fuel tank sock filter obstructions / clogs, the regulator, and the return line. It's a pump volume test. You connect a hose to the rail Schrader valve into a container, turn the IGN on, and run the pumped fuel into the container for a calibrated time vs. fuel volume pumped. I know the specs for the HOs, have done the test a few times when I suspect a fuel delivery problem, and it works well so you can move on to other possible causes.

 

No idea if the Renix system has this test documented though. Probably not.

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Just tried swapping in a known good MAP sensor...   Still not running well....  Haven't had a chance to try the fuel pressure test.  BUt I'm thinking that the pressure might be good enough because I can get it to run up over 3000rpm.  So it should be adequate to provide enough fuel at that rpm.

 

Any other ideas?

 

THanks

Ryan

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  • 2 weeks later...

Ok, borrowed a fuel pressure tester...   Hooked it up and turned on the key..  The fuel pump turned on and it read about 10psi.   I then attempted to start it.  After pumping the pedal like a mad man, I got it started and by pumping the pedal managed to get it to run at about 3000rpm.  The pressure gauge only read about 12-15psi, maybe a small spike up to 20psi but not consistently.  So is this a sign that my pump is bad???  

I'm just thinking now that this reading was a regulated fuel supply, maybe I should disconnect the pressure regulator and try it again or was this test good enough?  I also heard of pinching off the return line to check to see if its leaking past the regulator and not maintaining pressure... Thoughts???

 

 

THanks

Ryan

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Are you certain the fuel tester is working properly? Do you have another vehicle you can use the tester on ? The reason I say that is I went to my local oreilys and rented one and it was not reading correctly. You also said that it's running" very very very rich" . If you unhook the MAP sensor when it's "running" do you notice

any change ? Mine blew the MAP hose

connection off at the throttle body and gave all the symptoms you're describing .

 

You can also try bypassing the two wires on the white ballast resistor on the drivers side inner fender beside the airbox to see if that changes the fuel pressure reading .

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Few things come to mind to check out on top of what's already been said above:

 

1) have you checked the timing chain? Make sure it's not got too much slack

2) fuel pump ballast resistor; had a similar problem in a 1989 Cherokee I bypassed it

3) test the throttle position sensor

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Few things come to mind to check out on top of what's already been said above:

 

1) have you checked the timing chain? Make sure it's not got too much slack

2) fuel pump ballast resistor; had a similar problem in a 1989 Cherokee I bypassed it

3) test the throttle position sensor

2) first.

 

Unplug the TPS and see how it runs. The flat 3 wire connector.

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Are you certain the fuel tester is working properly? Do you have another vehicle you can use the tester on ? The reason I say that is I went to my local oreilys and rented one and it was not reading correctly. You also said that it's running" very very very rich" . If you unhook the MAP sensor when it's "running" do you notice

any change ? Mine blew the MAP hose

connection off at the throttle body and gave all the symptoms you're describing .

 

You can also try bypassing the two wires on the white ballast resistor on the drivers side inner fender beside the airbox to see if that changes the fuel pressure reading .

 

The tester is pretty much new.  Only been used once before(its my brother-in-laws), so I can't see it being faulty.  And I don't have another vehicle to test it on at the moment, I've stolen a bunch of parts off of it to test on this one so its not running.  But I might try later on if I'm not making any head-way

The MAP sensor is good as I swapped it with another from my parts truck and both were reading the same volts and pressure, so says my scanner.  And both vacuum line connections are good.

I tried bypassing the ballast, no change.

 

 

 

Few things come to mind to check out on top of what's already been said above:

 

1) have you checked the timing chain? Make sure it's not got too much slack

2) fuel pump ballast resistor; had a similar problem in a 1989 Cherokee I bypassed it

3) test the throttle position sensor

2) first.

 

Unplug the TPS and see how it runs. The flat 3 wire connector.

 

Tried the TPS, no change.  The scanner does say its reading reading perfectly at closed throttle and at WOT.

 

Do you really think its my timing chain?  To be running perfectly and then not.. if it was stretching, wouldn't it start running bad long before it quits running, and not just instantly, unless of course the chain broke in which case it wouldn't run.

 

And in case you're wondering, my scanner is a Snap-On MT2500 which gives me live readings off all my sensors. So if you want specific readings from certain sensors I can give you that, or if you give me readings that I should be looking for.

 

Thanks

Ryan

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It isn't your timing chain. It's low fuel pressure......

Alrighty then...  A new fuel pump it is!!!

That will have to happen after the long weekend..  Got a ball tourney to try and make it through first!!

 

Thanks for all the help

Ryan

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  • 1 month later...

Finally got around to replacing the fuel pump...  and low and behold... it RUNS!!!!!     Thanks for the awesome help guys!!!! 

 

With the old pump in there, initial turn key fuel pressure was only like 10psi, but with the new pump, it immediately spikes to 30psi..  

 

The new pump is about half the size of the old one, so it required a spacer and different rubber isolators to fit the bracket.  And what was in the tank appeared to be the original pump as I had to clean off years of road grime and undercoating just to get to the hose clamps.  I also had to stick my hand into the tank and fish out the return line rubber locator.  Just barely got my hand back out!!  That is one small and sharp hole!!   Its nice that I didnt have to drop the tank, the problem I had was that my air tanks were in the way, so it made it a little more difficult trying to tetris the pump out of the hole and get it back in!!

 

But at least shes running, so I can get back out wheeling!!

 

Thanks Again

Ryan

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