Hello all, I need some serious help. I have a 95 Jeep Grand Cherokee Laredo with the 5.2 MFI in it. I had to go behind a shade tree and replace all the gaskets. The shade tree decided to rebuild the motor and changed the timing chain and gears and use 4 large tubes of silicon from Lowes to seal the motor. ( and that's not a joke) I have replaced all the gaskets, (heads, intakes, exhaust, throttle body, and valve covers), I also replaces the map sensor, all vacuum lines, I had to clean the throttle body to remove the silicone from the vacuum ports. These guys covered every gasket completely with an inch of silicon, and I do mean every gasket. I cleaned the block and put the motor back together, I had the distributor positioned at #1 and the crank timing mark set at TDC. When you start the truck, it is running super rich, and it will idle for a minute or two and when you give it gas it stalls. If you move the distributor either way, the engine idle doesn't change, and when you give it gas if it revs up and doesn't die, it is spurratic and rough and will pop back thru either the exhaust or intake and you get the pinging. you hear the throttle body screaming like there is a vacuum leak but there isn't one. I have checked the tps and it functions correctly, the crank sensor works perfectly. So now I am at a loss. Nothing I do is making things any better! Can anyone please help me!!! I am ready to shoot this jeep!
I have also replaced the plugs, wires, distributor cap, rotor button, and checked the fuel pressure, it holds steady at 38-42lbs.

 

My first guess would be that your ignition timing is badly retarded - you've most likely installed the distributor out of time by 1 gear tooth. That's not hard to do, and not extremely noticeable unless you've seen it all before. If this is the case, you may have more opportunity to take some of your engine apart - running this far retarded (1 tooth out equates to a large timing angle) usually causes burned valves very quickly. I would get a step-by-step installation guide for that distributor, re-install it, and hope for the best. If you get the timing right and the valves are good, you should have no problem with the engine. If the valves are toasted (a light toast may not even be noticeable with a general compression test - you'd need to do a leak down test as well because combustion pressures are a lot higher than static compression pressures), you'll have problems with the engine under load (accelerating, or pulling up a hill as an example) - you'd have a general lack of power and often backfire through the throttle body.

 

Alfons, Thanks, I will definitely pull the front off to check the timing marks. I was wondering if they were off slightly. What is really funny is the motor seems strong, has good compression and the longest it has run is about five minutes at an idle (only once), but mostly will start then die. Any tricks for stopping the steel water line coming from the water pump from leaking? I don't want to do like they did and fill it with silicone.

 

First off, the dist DOES NOT control the timing but controls the fuel syn to the fuel injectors which is why you're running pig rich. The timing is controlled by the PCM. The fuel syn has to be set using a DRBII scan tool or can be set using a Snap On MT2500 scan tool. This can not be set by 'guess' but should be set as close to +4 as possible.

 

A distributor distributes spark to the appropriate cylinder at the appropriate time. The basic time for this spark is determined by the gear connection between the the distributor and (usually) the oil pump (the pump is usually synchronized with the rotation of the crank). If you have a 6 cylinder engine, there are 6 valid "basic" positions (only 1 is described in the manual) and if you have an 8, you have that many basic positions, and again, only the one set at the factory is described in the manual. These basic positions are fixed - this position, without load, is the basic firing position for the spark per cylinder. From this basic "spark timing" position, the changes to advance or retard the spark, based on sensed load, are made by the ECU, but the ECU CANNOT make gross changes, so if the basic position is out by 1 distributor gear tooth, you would be out by over 20 degrees on a 6 cylinder engine (all you need to do is count the teeth and do a quick calculation to see what the angular change would be per tooth) - the ECU can't compensate for this. The ECU also relies heavily on the input from the cam position sensor (normally located inside the jeep distributor) for it's final timing calculations, so your fuel injection timing will obviously also suffer.

This is a '95 engine with an OBD1 ECU and there was a lot more mechanical interaction and synchronization than you'll see on a more modern engine.

 

The possibility of being off a tooth is very possible but the symptoms the OP is describing is an extremely rich fuel mixture which in turn is driving the O2 sensors crazy causing the backfiring thru the TB and the high idle. The dist controls the synchronizing of the injector pulses to the firing of the cylinders.

 

Bwdakrt, it's not easy diagnosing odd symptoms remotely and I might be wrong in my diagnosis - I have been wrong before , but from my experience with the older jeeps, the symptoms fit a screwed up overall timing situation which, on these engines, was often caused by someone tampering with the distributor position. The TB backfire in this situation is caused by the overall bad timing situation where the spark & fuel injection is just out of step with the engine rotation & this can lead to a slight "ignition" in the intake manifold - this is what gives you that big "pop" and even have flame shooting up from the TB if it's out of step enough.

The valve problem arises from the "out of sync" condition as well. The combustion is still taking place when the engine rotation starts to open the exhaust valve allowing part of the combustion to exit into the exhaust manifold - this quickly overheats the valve stem, and often will cause the 6 cylinder jeep manifold (it's not cast) to turn red-hot. It doesn't take much running in this condition to warp the valves.

 

I'm not here to argue just offering another POV.

 

Well here is the latest... I replaced the timing chain and gears. Everything is lined up exactly by the book. Engine turns over, will not run and back fires thru the exhaust sounding like a cannon going off. If I rotate the distributor 180 degrees the darn thing will idle but not take any fuel!! I am totally at a loss now. So does anyone have any ideas??? again it is a 95 grand Cherokee Laredo with the 318 fuel injected motor.

 

This may be a given but was your #1 piston at TDC on the compression stroke when you replaced the timing gears?