dschappell

Summary: had rough idle, cleaned up IAC, got better, a week later, p0303 and 304 came up. Went away for a day. Came back next day. Replaced dist. Cap, rotor, wires, #3 and 4 plugs. Drove for a day, today the 0304 came back. So tomorrow I'm going to pull the plug and put in a new one. After that, if it returns, compression test? Everything I have done on this jeep has been my first time doing it. I'm not scared to really get into the engine, but I have to go very so and with the most knowledge possible.

Let me know if there is something else I can check first. I figure a 5$ plug is a cheap test to do since they can and do fail.

Thanks!

CCKen

When you pull #4 plug take a look at the firing end. See if it's fouled in any way. See if it's sooty, white. oily, etc.

It may be a good idea to do a compression loss check on #4.

If you don't have a compressor rent one from a rent-all joint.

Apply around 80-90 psi to the cylinder while its at TDC compression.

Listen for air escaping through the TB - burnt/warped intake valve. Air escaping out the oil filler cap or oil dipstick tube - bad rings. Air escaping out the exhaust tailpipe - burnt/warped exhaust valve.

Valves may have a piece of crap lodged under their seat. With suspected valve problems you can run a can of Chrysler Combustion Chamber Cleaner (Dealer) through the TB to see if it helps.

If there is no air escapng anywhere, you may want to pull the valve cover off and check the valve springs.

Let us know what you find.

dschappell

I'll give it a check tomorrow. Thanks. I'll post back

CCKen

Forgot to add: If you remove the spark plugs on either side of the cylinder you're checking and you feel air escaping out one, or both, of those plug holes, you may have a blown head gasket.

dschappell

Just drove it home, check engine light never came on. Lead me to think there is crap in there instead of something more serious like a blown head. Is that a fair assumption? I'm still doing the test tomorrow, but does the logic make sense?

dschappell

I did the seafoam treatment and plan to still do the compression test. The idle is much much much better and RPM is holding very stead at 660ish. My light is on and codes are now reading 300 303 and 304. The engine doesn't sound like it's skipping/hiccuping or anything. After I finish the comp test I'll post back

dschappell

So I still havnt got the test done, local stores don't have them for rent. I replaced the plugs 3 and 4 with the Bosch iridium 4 prong plugs and the CEL is no longer burning. It sounds fine as far as I can tell, no major jumping or shaking. Could I have simply had 2 failed plugs??? Seems very strange to be that close of a coincidence to me... Any way, problem isn't there from what I can see.

CCKen

With Bosch iridium 4 prong plugs in there I'm sure you'll be back on the forum.

But glad it's working okay for now.

Cheers

dschappell

I havnt had the check engine light come back on yet, but I did get my compression test tool today in the mail. Next week sometime I'll get out and check them.

I'll post back my findings.

Kalali

The test described in the earlier post is cylinder leak down test and needs an air compressor with a proper fitting for the spark plugs, etc. What you have is compression test gauge to measure cylinder compression. The compression across the six cylinders should read 120-150 psi with no more than 30 psi variation across the board. But aside from all this, your issue could also be caused by the injectors. Swap cylinder 3 and 4 to other injectors and see if the code follows.

dschappell

Ok, I have an air compressor so if this piece fits from the compression tester, could I use it for the leak down test?

billthedrummer94

What year jeep is this?

'90Cherokee

You can use the hose to do a leakdown, just remove the valve in the other end of the hose with a tire valve tool. Don't loose the valve, altho it looks just like a tire valve it's different and the tester won't work right with a tire valve.

dschappell

99 but it has a older engine in it. 1997 if I'm reading the tag right. It still has the distributor for spark plugs.

billthedrummer94

This might be off base but check the head casting number. The stamp is on the drivers side, top of the head in between the exhaust hookups for three and four. You might have to pull out the silver heat shield. Make sure it's a 0630 head casting. If it's 0331 that could be your problem. The 331's are prone to a crack in between 3 and 4.

Just a thought.