Engine miss at operating speed

I have a 1987 Jeep Cherokee with a ton of miles (217,000) that has developed a miss when just cruising along at normal speeds. When you put the pedal to the metal, it has all the power you want and then some. The number 4 cylinder seems to go dead - even at idle. I've checked the injector fire from the computer, changed the injector (swapped #3 for #4), changed plug wires, changed the plug, new rotor, new cap (has good spark),checked the compression (150), checked the fuel rail to make sure fuel is spraying and checked the intake manifold gasket (sprayed starter fluid around the #4 intake to check for leaks). None of this seemed to help. I even put on a new fuel pump. I even stooped low enough and took it to the local Chrysler dealer to have them scan it for codes....they told me there was a problem with the #4 cylinder...duh! Didn't know what the problem was just that there is one. I hate to give up on the old girl but.... Anyone have any other suggestions? At this point I'll give it a go. One thought that I had was that the intake manifold may be leaking internally. I've never had one of these apart. Is there a water jacket located next to that cylinder? Thanks.

I think you said that #4 has a good spark. How and where did you check for this??

Sounds like you've covered most of the obvious things... What about the EGR valve?? Not sure how it would consistantly affect only the one cylinder but......

If there was a problem with a water jacket you would likely have a cooling system problem too.

I'll keep thinking on it and maybe someone else will jump in too.

JD

The local guy who is the resident "if he can't fix it no one can" has a spark tester that we put on that cylinder. Checked EGR valve. Seems to be functioning. He's stumped too. I am loosing antifreeze somewhere, although I did find a small leak in a hose, but can't run the vehicle far enough to see if that stopped the coolant loss. Thanks.

Just some old fashioned stuff here but have you done a compression check on the engine...especially #4? In lieu of the compression check, a leak down check would reveal a lot about the condition of your motor. Hook up a vacuum gage and you can find the health of your valve train (might be a sticking valve). You say you suspect a coolant leak...have you done a pressure check of the cooling system? Is water getting into the oil..or vice versa? At 217K, it's a safe bet to say your motor is probably a bit "tired"......

mike

Checked the compression (150). Did not do a leak down check, but will give it a go. Not sure about the coolant leak. It has the '87 CCV syndrome - oil blow back into the air filter and it sometimes appears to have a watery base. However I have faithfully changed oil every 3k miles and never seen a hint of coolant in the oil. I would normally have to agree with you about being tired at 217k, but if you put the pedal to the floor it has all the zip you want. I'm leaning to agree with you about some kind of sticking valve or internal leak on the intake manifold gasket. Thanks for the suggestion! I'll give anything a try at this point. Let you know if I find out anything.

When you check the compression, do all the cylinders. The lowest reading should be no less than 75% of the highest reading (I think that's the right number.... been awhile) on a healthy engine. Also, if two adjoining cylinders are reading low.... might indicat a blown head gasket between them.

Oil mixed with water will have a milky brown look to it

A sticky valve will show up in a vacuum gauge reading......

mike

Checked the compression (150). Did not do a leak down check, but will give it a go. Not sure about the coolant leak. It has the '87 CCV syndrome - oil blow back into the air filter and it sometimes appears to have a watery base. However I have faithfully changed oil every 3k miles and never seen a hint of coolant in the oil. I would normally have to agree with you about being tired at 217k, but if you put the pedal to the floor it has all the zip you want. I'm leaning to agree with you about some kind of sticking valve or internal leak on the intake manifold gasket. Thanks for the suggestion! I'll give anything a try at this point. Let you know if I find out anything.

try this for your 87 ccv syndrome,
http://www.automotiveforums.com/vbulletin/showthread.php?t=432608

Did a compression check on all cylinders. Range from 135 - 150. Going to try the vacuum check. Thanks for the reply!