Hey everyone. My poor old jeep is having some issues. Here’s the stats: 94, 4.0 engine, 240k miles (jeeps am I right?). It started overheating on me so I start with replacing the thermostat. No dice. I noticed it was “weeping” coolant, so I replaced the water pump. No dice. It’s still running hot and throughout the process of replacing things and test driving it is now struggling on accelerating/giving it gas to switch gears. It even out right died a couple times. (Air filter was gunky, replaced it)
So I’m at my wits end and hoping you fine folks with more knowledge than I can help me get my old girl running again.
First job is to determine why it’s overheating. Other possibilities are plugged radiator, blocked hose, or leaking head gasket. Check the head gasket first before you start replacing parts. With the engine cold, remove radiator cap and start the engine. Have someone rev the engine while you look for bubbles in the coolant. Bubbles = head gasket leak. You can also secure a plastic bag over the filler neck with a rubber band, squeeze closed the overflow hose with vise grips, and see if the bag inflates. Or you can do a more scientific test and have a mechanic use a chemical test kit on the coolant to determine presence of mildly acidic CO2 from a head gasket leak in the coolant. You can also buy a test kit and do it yourself.
The low power/running poorly also suggests low compression from a blown head gasket. You can have a mechanic do a compression test on the cylinders and a leakdown test on the cooling system.
If you can verify that the head gasket is not leaking, then you can investigate the other options. But with that many miles on the engine, you may not be that lucky.
And apparently these engines can blow a gasket between cylinders without leaking coolant. So even if the cooling system is intact, the gasket may be leaking between cylinders. Again, have a mechanic do a compression test on all cylinders.
The problem might be with a restricted catalytic converter. Your Jeep is an OBDI vehicle. So there’s no O2 sensor after the catalytic converter to indicate if there’s a problem with the cat.
And if the cat is plugged, this can cause the engine overheat, and the engine will lack power and stall at times.
Find someone who knows how to test for a restricted exhaust system.
Okay so I have an update. I haven’t had the chance to check the head gasket until today. Had my wife rev the engine for me while I looked for bubbled and I didn’t see any. Also tried the plastic bag trick and it didn’t inflate.
Also I had her hop out and head back inside so I could monitor how it was running and she shut the jeep off. I hopped in and tried to start it and it would crank but not turn over and it had a bit of a gas odor to it. Does that shed any lights or ring any bells? Thanks again for all the help.
While your basic checks make the head gasket less likely I wouldn’t write it off yet. I would want to check the engine’s compression . If you don’t have a gauge, the auto parts store loaner tool programs often have them. They also often have block testers which are used to check coolant for combustion gases.
But you should worry about an exhaust clog as @Tester suggested. If you’re determined to DIY the most straightforward thing to use is a vacuum gauge. A quick and dirty shade tree method would be to temporarily remove the O2 sensor and see if the power problems are any better. The O2 sensor hole just provides a place for exhaust pressure to escape.
Heads gaskets don’t often fail on that engine but I repaired one that showed up with identical symptoms and after ruling everything else out pulled the head and found the gasket failing.
If you pull the head get a good shop manual and follow the instructions. Be sure to coat the front manifold bolt threads with high temp sealer.