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Al

Help! Fix A Jeep for Me For Christmas! Puzzle!

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Al
Here is a riddle for you. This is a problem I have worked on and do not have an answer for. The details follow. My son has an 89 Jeep Cherokee with a 4 liter 6 cyl. He put in a rebuilt long block 2 yrs ago. Had 30,000 miles on it and it started to miss on #6. He replaced the plugs, wires, rotor, cap and as part of tune up air and fuel filter. Took it to a shop and had it put on an analyzer. Compression on 6 was bad 70 lbs. Came and talked to Dad. We pulled the head and sent it to a machine shop. It is a shop that I use a lot and they do very good work. They called and said they wouldn’t fix it with the valves that were in it. It had had the guides reamed and had oversized stems, which I am comfortable with. They would only fix it with new guides and valves, I told them just fix it, I don’ t have time to pick it up and take it elsewhere. New guides, valves, springs, valve stems ground to correct height, springs shimmed to correct height. Put the head on, compression 142 to 145 all cyls 145 on 6. Put it together, still misses. Pull the wire on 6 and no change all the rest effect it. Spark jumps over ½ inch. Spark advance works properly Swapped 6 and 3 plugs. Problem still 6, swapped the wires, still 6. Went out and talked to a mechanic I have a lot of faith in. He had a Jeep just like it sitting there that ran good, but the owner was mud running with it and covered the radiator with mud and since Bud was driving, he kept running it as it overheated. He said we could take anything off the Jeep we wanted. Went out and pulled the fuel injection rail off, got some new o-rings and changed the #6 injector, no help. Rechecked everything and went to the shop and got my oscilloscope off the alternator bench and looked at the signals on the injectors. All were the same and the duty cycle changed correctly on each one as the engine was accelerated. Went to parts house, bought a new injector and put it in 6 no help. Hooked a Snap on light to the injector line and all of the injector leads check fine. Swapped the wires to 5 and 6 injectors, to see if they might have gotten switched, no help Checked the fuel pressure and it is good. Pulled the injector rail out and the all injectors spray equally. Disconnected all vacuum lines and plugged no help. Went to the old Jeep and got the computer, swapped the computer. No help. Looked at all the injectors signals with the scope again. All check OK. Pulled the valve cover and checked the lift of all of the valves. All equal. Sprayed WD40 and then carb cleaner all around the manifold checking for leaks. No help. Talked to another local guru and he said he had seen a V4 Ford engine that had a camshaft broken on the back cylinder that the bumps on the break would turn it, but the valves were out of time. Went home and rechecked the valve lift and the valve timing. Valve timing is good so that is not it. Put valve cover back on and pulled the intake manifold off again and looked very carefully at the gasket. Everything looks perfect. Looked in the ports under the injectors all clear. NO CRACKS. Put it back together. Ran worse. Wouldn’t hardly idle. Acted like EGR valve was stuck. Removed it was. Replaced it again runs well but still has miss on 6. I have shared this with about 7 mechanics locally with a total of probably 100 years experience. No one can give me an idea of where to go. Am going to get out and back on an analyzer again and see what it says. Any of you that are car literate and have any thoughts on this I am interested in anything I may have missed. Maybe this will give you something to think about while it is cold and snowy outside. With all of the time I have in this maybe I might have been better off to donate him a different car. It just ticks me off that I have this engine that has everything it is supposed to have and I still can’t fix it. Send me a Christmas Present, fix it. Thanks and Merry Christmas. Al Eden

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badcars
just the opinion of a former mechanic of 15 years. it si like this suck, squeeze, bang, blow. sounds simple put can drive you crazy. if you have 145# on 6 , and have fire on six at the correct time(very important) you have a cam problem. with the engine running install a vaccum guage and monitot, does it jump eratically, then it is a camshaft problem if the cylinder itself is sealed (rings and such). i would get a dial indicator and check the total valve lift at the rocker, compare it to the other healthy cylinders. thjsi is to be done only if you have fire at the correct timne at the cylinder. you probably have a hall effect distributor and if the air gap is different on the #6 cylinder throw it in the round file and replace it. keep me posted give more infoprmation, i would love to help if i had more to go on! by the way i do build alot of engines, do a leakdown, i do not trust a compression test, just my personal opinion.

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Al
Hi, We did change the computer and checked all of the camshaft parameters. Lift and valve timing. In the list, we tried to sequentially close all the doors. Its just this darn mouse hole thats got me. I did not change the distributor, but that is an excellent suggestion. Al

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Mel_W
Al, If this year of jeep has the charcoal canister with three hoses it could be a blown canister. Use a snap clothes pin to clamp off the lines and see if this does anything. The next thing I would check is the voltage at the injectors, As a small difference in this can change the timing and spray late. Also an elcetrical leak from a cracked wire could be picking up harmonic interfearence and change the pulse. With the plug removed from #6 and the engine running do you get raw fuel out of the plug hole? I don't know the answer but I will help ask the questions. Mel

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Al
Hi, We blocked all of the vacuum lines including the brake boosed. We used the Snap on test lights on the injector connectors. and checked both the signal shape, pulse width and amplitude with and oscilloscope twice. On other thing the plug in 6 is dry, but black and almost sooty. I am thinking that the comment regarding something strange in the distributor might be a good place to look. It is the only thing that we haven't REALLY tore into. It acts like almost like an interchanged plug wire, but I have checked them at least 10 times. Al

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jkmustang1
The shop that did the head did they check it for level flatness. It could be a fraction warped not seen by eye but bent. All though from all the other testing you listed I am leaning to the distributer.

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ReedS
GM's of this era used to have problem with the distributor shaft mounted rotor (the one that was used for ignition computer) developing cracks. When this happened it would cause a rough running eng, cracks were hard to see until you knew what you were looking for, most of the time occurring at the rivets. I know the jeep isn't quite the same but could be a similar problem and since the dist has been a part that hasn't been changed I agree that it would be an excellent place to take closer look. Does this eng also have a crank shaft position sensor? If so may be another place to look.

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patrician12
The distributor on 6 cylinder Jeep only distributes the spark,spark timing comes off the flywheel magnetic sensor.The Jeeps use a French Renix system that is a hold over from the Renault days.Compression test are not always reliable test for run.A car with a clogged exhaust will show great compression but won't run.Could you have a valve that is too tight.I believe you are missing something so basic here.Inject propane thru rear vacuum hose and see if miss goes away.If it does you have a fuel injection system or a fuel problem.If it doesn't it is a mechanical problem. In the over 30 years of being a mechanic I never saw an Amc with a bad camshaft.

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markpogue
In your post you stated that first the plugs, wires etc were changed. You then found the low compression, I would put a new plug in #6 and see what it does. The original miss may have been caused by low compression and a defective plug could be causing the problem now. I would not put much faith it the fact that the plug will fire out of the engine. I have a old Champian spark plug cleaner and after cleaning the plug it has a spark test chamber. You screw the plug into the test port and turn on the spark. you then add pressure to the chamber with compressed air. You watch the spark through a viewing window and you would be amazed at how many plugs fire well until under pressure. I have seen everything from a intermitent miss to no spark at all.?

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Al
Hi, Plug has been changed twice since reassembly. Plug wires, plugs, cap and rotor were changed by son before shop said compression was low, before I ever looked at it. All of the wires including #6 will jump 1/2". I think you distributor guys are headed in the right direction. As I said we pulled the FI rail and turned the engine over and all injectors sprayed the same. As soon as I get a chance to get it in, I will swap the Distributor from the other Jeep and see what happens. Thanks, Al

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iweld
You mentioned that the valves were re-done. You mention that the comp. is good. You mention that the valve lift is correct. Is it possible that a valve isn't going back shut fast enough? This would cause a miss and still give good compression. I would lean toward the exhaust valve because of the sooty plug and the fact that the exhaust valve gets hotter and swells more. I'm not sure how you would check this out, but marvel Mystery oil in the crankcase and the fuel would be a cheap try to see if the valve "frees up" and if the "miss" goes away after half a tank of gas I would say the valve is/was the culprit.

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HubbardRA
Al, Check for a cracked exhaust manifold near the rear cylinder. This can affect the signal to the oxygen sensor. Manifold broke on my daughter's Jeep and we replaced it with a steel tubular one. No exhaust manifolds in the junk yards around here because of the breaking problem.

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UCD
Al I just thought of something. Check the intake manifold for cracks or air/vacuum leaks between the #5 and #6 intake runners. Check the gasket and between the runners in the head between #5 & #6. I have run into this before, can't say why it will only cause one cylinder to skip and not the other.

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a7117puller
1986 jeep j20, 360amc, auto tranny, dana 44, dana 60, ....plow truck.......spring project...too much to do to it, and picking up a '77? j20 w/401amc to use for parts. Anyone got a J-series truck sitting somewhere yall be willing to sell?

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Salthart
Al, Slow down a bit and look at what you DO KNOW.. Your getting fuel, and your getting fire. You have compression ( but didn't say if it was checked at WOT or not ) Being that this is a EFI, a vacume leak won't act quite the same as a carb engine. But you stated that the plug was wet and carboned.. Crazy as it may sound, I'd look for a grease rag in the #6 intake runner..LOL.. Really sounds like your Starving for air on # 6 don't it ? If that don't work, drop the exhaust manifold and start it.. Just about has to be one or the other IMO ( which don't mean a whole lot)..LOL Good luck bud...........................

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Salthart
Something else just came to mind, And forgive me, I know its simple and not something you'd be likely to miss. But you do have the option of looking at it while I'm just going on what your telling. When you said the plug was wet, Are you quite sure its wet with fuel and not maybe antifreeze ? With all the head work etc there is that chance... A crack missed by the machine shop or a leaking gasket etc.. Again, its just a stab in the dark..Good luck.............................................................

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Al
Hi, The plug isn't wet, but gets black carbon on it. When we removed the manifold the second time it was to check for any cracks, leaks etc. plus check the hole from the injectors to the manifold. Looking at the plug it acts like it is sort of firing. Removing any of the other plug wires causes the engine to run worse and miss on 2 cylinders. Removing #6 has no effect. When we had the manifold off the second time, after we had sprayed WD40 and carb cleaner all around it with no effect, we carefully checked the gasket for imprints, looking for leaks. We also had capped off all vacuum lines at the manifold. I kept looking for a vacuum leak as that was what I expected to find. It acts like it had wires crossed. I have checked that 10 times. I don't have it here right now, but am going to get the distributor from the other Jeep and see if it has any effect. The damn thing has everything it needs to run. It is 15 yrs old and like a lot of teenagers it doesn't do what it is told. Believe me I have told it plenty. Thanks, Al Eden

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