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Unread 10-05-2011, 04:58 PM   #1
Diamond-x
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Symptoms for a bad relay

What are the symptoms for a bad fuel pump or ignition relay?

I was getting on the freeway this morning and my 1991 4.0L Cherokee started popping(back firing) and quit. It cranks and cranks but wont fire up. I got it towed back to my driveway with AAA coverage.

I have recently changed my TPS, CPS, Plugs, Plug wires, Cap and Rotor and my O2 sensor in an attempt to resolve a bogging condition while under load.

I have also installed a new battery, battery cables and cleaned all the grounds (On the side of the block, near the battery and near the ECU).

I also added fuel treatment in case it was a case of bad gas.

Prior to this morning my fuel presure read 20psi at idle and just under 40psi when accellerating.

I think the only sensors left are camshaft position, MAT, MAP and water temp.

After it quit, while cranking the motor this morning I noticed the oil pressure guage wasn't resistering, which is a new symptom. I also turned the key on/off a few times and noticed a few codes (76? and xx?) I'll post those when I get home from work.

We have a trip planned this weekend, so I hope I can get it fixed in the next couple of days!

I've searched the ***t out of this one, so please respond accordingly.

Thanks!

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Unread 10-05-2011, 10:00 PM   #2
filluptrk
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I believe your fuel pressure should be much higher than 20 no matter if it idling or not. When a relay goes it goes period. Then whatever runs off of it won't. It sounds like a fuel delivery problem and a lean condition that shows up in the popping you are experiencing. I am not that up on 91's so someone here may be able to give you the proper specs.
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Unread 10-06-2011, 06:05 AM   #3
cruiser54
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Quote:
Originally Posted by filluptrk View Post
I believe your fuel pressure should be much higher than 20 no matter if it idling or not. When a relay goes it goes period. Then whatever runs off of it won't. It sounds like a fuel delivery problem and a lean condition that shows up in the popping you are experiencing. I am not that up on 91's so someone here may be able to give you the proper specs.
Your fuel pressure is low. Pull the vacuum hose off the fuel pressure regulator and see if there's fuel present. Shouldn't be. If no fuel there, I would suspect a weak fuel pump.
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Unread 10-06-2011, 06:14 AM   #4
unkajeffy
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code 76 is the fuel pump by pass resistor relay open or shorted condition. That relay hangs upside down next to the power distribution center on the engine side. It may be the resistor itself, little white car looking thingy on the drivers side fender inside the engine bay. Take the wire off the resistor and jumper them and see if that solves the problem. Then re-check the fuel pressure it should be 31psi at idle with the regulator plugged to vacuum. 39 with vacuum off the regulator. That resistor goes bad and can cause the problems you are having. You can replace the resistor or permanently jumper it though you might get some electrical noise thru the radio. Mostly on the am god shouting channels though.
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Unread 10-06-2011, 06:18 AM   #5
unkajeffy
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Also carefully check and clean the grounds and the battery cables and make sure they are tight.
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Unread 10-06-2011, 01:47 PM   #6
Diamond-x
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Thanks for your responses, I'll definitely re-check the fuel pressure.

Now for the real cause, I pulled the distributor out to check the drive gear. In the past I've read about some funky ignition sysmptoms based on a cracked or failing distributor drive gear. The distributor gear was completely worn away on one side. I got a new distibutor, I'll be dropping the pan and attempting to get all the metal shards out before they destroy my motor. A check of the camshaft showed no problems with the drive portion, the distributor gear is sacrificial.

I bought a few cans of brake cleaner and will be spraying liberally to clean the crankcase. I'll then refill with oil, use a new filter and run it up to operating temperature, then change the oil and filter once more.

Hopefully I get it all.
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Unread 10-06-2011, 02:22 PM   #7
cruiser54
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The shards can't get through the oil pump screen.

Drain your oil. When done draining, run a sacrificial quart of fresh oil through and let it drain right through. If you're really worried, put a magnet at the bottom outside of the oil pan like automatic transmissions have on the inside of the pan.

I'd still check that fuel pressure again. Your pressures were low.
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Unread 10-06-2011, 04:07 PM   #8
filluptrk
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cruiser is right. That pan is not exactly a lot of fun to get off. The magnet idea will work, but get a super strong one, and old speaker magnet is great.
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Unread 10-06-2011, 04:17 PM   #9
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Old hard drive magnets are really strong, but any decent magnet like one from an auto trans pan would work fine.
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Unread 10-07-2011, 09:08 PM   #10
Diamond-x
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I pulled the pan and wiped it clean with brake cleaner, as well as what i could reach in the crank-case valleys and channels. Put the pan back on and refilled with oil and a new filter. Tried a few times to get it started but the battery was dead before we could properly index the distributor.
There was a lot of fine shavings in the oil and a few large pieces.

Just did this a few months ago for my rod bearing.
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Unread 10-08-2011, 02:50 AM   #11
RacerX
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Quote:
Originally Posted by Diamond-x View Post
....
There was a lot of fine shavings in the oil and a few large pieces.

Just did this a few months ago for my rod bearing.
Hmmm....
Maybe that's what happened to your distributor, the rod (or bearing) shavings got in the gears...
I wonder what else may have metal shavings?
I would AT LEAST - with a fully charged battery, fill the oil pan, unhook the coil, and crank the engine for SEVERAL minutes!, maybe use Rislone to help rinse everything out.
Then re-change the oil.
You may also want to look at the timing chain, Drive-shaft bearings & seals and ... piston rings?
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