Any engine guys here?

[Forrest Price]

My 93 Jeep Grand Cherokee Limited has a rough idle..real rough....

So, I cleaned the TB with a can of 3m spray, replaced the ignition coil, tried a new battery, checked all the vacuum lines I can find and finally a compression check.

Compression on 2 front cyl's, passenger side are 65psi, and the front cyl had a sooty plug. All others are right around 125 or so with nice clean plugs. Engine runs right about 205-210 even with a/c full blast!

No oily residue in coolant, no water in oil. Nice and clean on both parts.

Is it possible the head gasket could just be blown between these 2 and not leaking oil/water?

Stuck lifters maybe?

Valve carbon buildup?

When I say rough idle, I mean, you could work kinks out of muscles at the stop light!http://www.jeepforum.com/ubb/biggrin.gif Give her some gas, and she smooths right out and away we go.


I figure, I'll go to Jeep tomorrow and pick up a can of whatever cleans the guts out of these things and see what happens, but wanted to approach the experts here and see if I missed something before I start yanking heads off!

[Don Baer]

Compression on 2 front cyl's, passenger side are 65psi, and the front cyl had a sooty plug.

Check for a leak in the exausht manifold gaskget on that side of the engine. leaking between cylinders.

[Anthony Anderson]

[quote=Forrest Price]My 93 Jeep Grand Cherokee Limited has a rough idle..real rough....

Is it possible the head gasket could just be blown between these 2 and not leaking oil/water?

Yep, this is very possible, and is probably the culprit.
Another thing to check is the distributor cap. Sometimes they will get a hairline crack that is barely visible, and is often overlooked.
Best thing to do when troubleshooting is start with the cheapest possible fix and go from there.If it is not the distributor cap, then I would take it to the dealer and have them run a scan on it, and see if any codes come up. If any codes come back, then immediately rule out the ones that would obviously NOT cause the problem (as dealers tend to give you everything that is wrong, and try to get you to fix everything for BIG $$$). If you cannot find anything then I would go to the head gasket. Ask the dealer if they have a borescope, and if they could look into the chambers of the two suspect piston cylinders. I would guess (without being able to look at it) that this is where the problem lies. Good luck and hope this helps. Regards, Bill

Be sure to check out that distributor cap and wires very closely, and keep in mind that often you cannot even tell they are bad by looking at them.


Stuck lifters maybe? Doubt it
Valve carbon buildup? Doubt this one too (but millions is spent every year trying to get rid of that "carbon")

[Forrest Price]

Hey that reminds me. We've got a borescope out at the hangar. I'll try to get it tomorrow and take a peek inside those cylinders and see if there is any scoring or other obvious signs.

Still, my hunch says head gasket. That low compression on the 2 cyls next to one another seems to fit.

I suppose it could have gotten hot and damaged the rings in those two.

Not sure about how the exhaust gasket would have an effect on the compression since it's external of the engine cylinders. Maybe I'm being thick here.
Exhaust being drawn in to neighboring cylinder maybe? but if the valves are seating properly, this shouldn't be an issue should it?

[Anthony Anderson]

[quote=Forrest Price]Hey that reminds me. We've got a borescope out at the hangar. I'll try to get it tomorrow and take a peek inside those cylinders and see if there is any scoring or other obvious signs. Forrest, look in the area of the head gasket where the head sits against the block. I would say that you will see a black area in the head gasket between the two cylinders where it has blown.

Still, my hunch says head gasket. That low compression on the 2 cyls next to one another seems to fit.

I suppose it could have gotten hot and damaged the rings in those two.
I doubt if you damaged the rings unless you ran it low on oil, and in that case there would be a drop in compression in most of the cylinders, further, the compression would be pretty much consistent in all of the cylinders. How many miles are on the Jeep?

Not sure about how the exhaust gasket would have an effect on the compression since it's external of the engine cylinders. Maybe I'm being thick here. If an exhaust gasket was blown, what you would expect would be a loud "backfire" sound as you let off the gas, after you accelerated all of a sudden. This is caused by sucking outside cold air in through the defective gasket, and rapidly mixing with the extremely hot air inside the exhaust manifold. The exhaust manifold would most likely have no bearing on your problem. Regards, Bill

[Stu Ablett in Tokyo Japan]

Usually on compression it will be one of three things, the head gasket, the piston rings or the intake and exhaust valves.

The quick and dirty test for valves or rings is to squirt some motor oil into the offending cylinder, don't need a ton, just a few squirts. Then, try your compression test again. If the test number goes up near the others, then you have bad rings, if the numbers don't go up, then you have a bad valve or head gasket.

I do not know much about that motor, I'll assume it is a V-8 because you say "Front two cylinders on the Passenger side", but it possible that the head gasket is gone between those two cylinders, and there is no water jacket to leak there, "Possible" but, my old automotive teacher used to say "When you hear hoof beats, think horses, not zebras" :D

I'd say you have a burnt valve, or bad rings, I hope it is the valves, that is somewhat less painful.

If you do get one head pulled to have a valve re-ground, it is my opinion that you should have them all reground, and you will get a new head gasket in the deal, also depending on the mile on the Jeep, you could also need new valve springs, as this could be the cause of the burnt valve, the springs getting weak and not closing the valves fast enough to cool them.

Good luck!

[Anthony Anderson]

Forrest I had a truck a few years ago that did the same thing. It was a four cylinder. The head gasket was blown between the front two cylinders. If there were no symptoms/signs leading up to this, and it happened all of a sudden, I would look to the head gasket.
Good Luck. Let us know what it turns out to be. Regards, Bill

[Roger Everett]

Forrest;
First a sooty plug is usually a sign of over rich mixtureor incomplete burning, assuming it wasn't wet.
The 65 psi reading is as you know too low when compared to others. I would check for burnt valve. When I was in the auto repair buss., I had a set of plug inserts, that had a male air fitting on them, you could also use a blow gun with something to seal it around plug hole. Remove one plug at a time, bring cylinder up to TDC on firing stroke ( both valves will be closed) ( can pull dist. cap and check position of rotor ), have compressor set at 50-60 psi., have an assistant pressureize cyl. . Go to tailpipe, if burnt exh. valve , you can hear air coming out of tailpipe or hold cig. to tp and watch escaping air blow smoke.
If is burnt intake valve you can check for air coming out of carb. or engine intake (with air filter removed).

Roger

[Forrest Price]

Forrest;
First a sooty plug is usually a sign of over rich mixtureor incomplete burning, assuming it wasn't wet.
The 65 psi reading is as you know too low when compared to others. I would check for burnt valve. When I was in the auto repair buss., I had a set of plug inserts, that had a male air fitting on them, you could also use a blow gun with something to seal it around plug hole. Remove one plug at a time, bring cylinder up to TDC on firing stroke ( both valves will be closed) ( can pull dist. cap and check position of rotor ), have compressor set at 50-60 psi., have an assistant pressureize cyl. . Go to tailpipe, if burnt exh. valve , you can hear air coming out of tailpipe or hold cig. to tp and watch escaping air blow smoke.
If is burnt intake valve you can check for air coming out of carb. or engine intake (with air filter removed).

Roger

That's on the agenda for this morning. We've got a cylinder leakdon tool we use on the aircraft engines during annual inspections. I'll try to find the correct adapter for the auto plug threads and run a cylinder leakdown test this morning to pinpoint the situation.

Still get the feeling that no matter what, that head's coming off though. :(

Oh well, at least I won't be bored this weekend! Scraped knuckles, here we come!

BTW, the Jeep only has 97,000 miles on it.

[Robert Mickley]

If its not an ignition problem, I would have to go with the other guys and say blown head gasket or burnt valves.

Roger
Neat idea, but if it is a blown head gasket isn't it possible the compressed air could flow through to the next cylander and still come out an exhust or intake port? Thats just kind of my line of thought on it. Of course using your approach you could pull the plugs on both offending cylanders. If the gasket is blown you would get air transfer between them.

Of course I've been wrong before, just ask my wife:D

[Roger Everett]

I had an auto repair shop for 17 years and was certified, and except for when there was a serious over heating, and once with a alum.head, I never saw a blown head gasket between two cylinders. usually it was from cyl. to water jacket, and you can sure see the boubles when the rad. cap was off , and they are usually caused by overheating. Also if you get a blown gasket, good proceedure would be to get head magnifluxed , check with a machinist straight edge and feeler gauages, and maybe have it surfaced a pinch just for insurance.
roger

[Joe Pelonio]

Ther are other things that it could be too, carboned up throttle body, easier to remove than the heads. By the miles on it I'd also consider the
catalitic converter becoming clogged, which would starve you for air at idle but run better at higher RPM. Even the crank sensor being bad, or a warped flywheel preventing the crank sensor from getting accurate info to the computer, or even the computer itself.

[Michael Cody]

My 93 Jeep Grand Cherokee Limited has a rough idle..real rough....

So, I cleaned the TB with a can of 3m spray, replaced the ignition coil, tried a new battery, checked all the vacuum lines I can find and finally a compression check.

Compression on 2 front cyl's, passenger side are 65psi, and the front cyl had a sooty plug. All others are right around 125 or so with nice clean plugs. Engine runs right about 205-210 even with a/c full blast!

No oily residue in coolant, no water in oil. Nice and clean on both parts.



I agree with blown head gasket or burnt valves, leakdown test will tell you, I am voting stuck/burnt valve.. if you having real bad luck then a cracked head. If you do pull the heads off have them checked for cracks unless the valve or head gasket is obvious.

[Jim O'Dell]

I'll add my guess as valve problem also. Won't be any of the ignition/engine management electronics, not for low compression. If this is a fuel injected engine, hold the throtle to the floor to shut the injectors off, and crank the engine. Should hear an uneven crank speed when it gets to the offending cylinder(s), kind of like an audible skip. If carburated, pull the ignition coil to do this. Next I'd pull that valve cover and have a helper crank the engine the same way, and watch the valve train at the offending cylinder(s). See if they are moving smoothly like the others, or if they are sticking, which "could" allow for the low compression. Your idea of the boroscope is good, but may not show a gasket problem...should show a valve problem. Compressed air check is also a good idea. Jim.

[Forrest Price]

Well, that was a fun day....not.

What a job. Man, you'd think Jeep could have designed a bit easier engine to work on! Who's the idiot that decided to put the distributor cap back THERE?! And let's just not discuss the stupid place for the oil temp sensor sticking up like a sore thumb, delicate as can be, begging to get snapped off! (Happy to oblige, $24.95 later!)


OK, so it was everything!

head gasket was bad in 2 spots by #2 & #4cyls. #2 injector was really grungy looking, and exhaust valve in #2 was really really grungy looking!

Took the head over and had machinest friend pop the valves out, re-grind, lap etc and check out the head. No other problems.

Soaked the injectors in Techron concentrate and cleaned what I could. Dumped a jug of techron in the gas tank and I'll run 2 or 3 jugs thru it, as well as some Marvel mystery oil or whatever is best for the block.

All back together tonight and test driven.

Idle is back say 95% smooth now. No more jumping around, but not perfect yet. Figure those injectors playing huge part in that, and most likely culprit to the #2 exhaust valve failure.

New plugs, cap, rotor, wires while I was at it.

Thanks all for the help. I'll keep tinkerin with it, as I really like the rig!

Time for a degreasing shower and bed! What a day. :thumbsup:

[Stu Ablett in Tokyo Japan]

Sounds like you got it licked!

so a combo of the head gasket and the valve, yep, that would do it.

Cheers!

[Jim O'Dell]

Glad to hear you found the problem, and better yet that you have the skills to tale care of it! Might want to do an induction serice to clean the carbon out of the other cylinder, unless you pulled both sides down and checked the valves while you were at it, but your post didn't sound like you did that. Get a good injector cleaner that you disconnect the fuel line and run through too. clean all the injectors that way, and I'd bet you have it licked. Remember to run the induction cleaner away from a neighborhood. That stuff smells nasty, and can leave a big white cloud when you start the vehicle back up! Jim.

[Forrest Price]

.... Get a good injector cleaner that you disconnect the fuel line and run through too. clean all the injectors that way, and I'd bet you have it licked. Remember to run the induction cleaner away from a neighborhood. That stuff smells nasty, and can leave a big white cloud when you start the vehicle back up! Jim.

Jim, I dumped the jar of Techron in it, and I figured on some Marvel for the crankcase. Not familar with the type of injector cleaner you mention. Could you give me some info? Sounds like a good deal (and maybe some fun too!):D

Thanks,
Forrest

[Stu Ablett in Tokyo Japan]

..............Remember to run the induction cleaner away from a neighborhood. That stuff smells nasty, and can leave a big white cloud when you start the vehicle back up! Jim.

You mean neighborhood that you like :D ;) :rolleyes: :D

[Anthony Anderson]

Hey Forrest, Glad everything worked out for you, and that your Jeep is back on the road. Now you can get back to woodworking-much cleaner. Regards, Bill