Rough idle, positive pressure from radiator with cap off...

Hi all. This is my first post on , so I will be as brief as possible in describing my problem.

I have a 97 Jeep Cherokee SE, 4.0 with 175K miles on the motor

2 weeks ago, I replaced my intake manifold gasket, exhaust manifold, and motor mounts. Ran fine for a 23 year old engine.

A week later, I pulled off the throttle body to clean it and replace the IAC and TPS sensors (was going to replace the MAP sensor, but O'Reilly had the wrong part in the box for that one). Ran great for a day or two, then I developed a really rough, but consistent idle. I already replaced the MAP sensor a day ago, and that seems to have helped to some extent. Ignition system uses Accel plugs, coil, wires, cap, and rotor if that info helps any.

Note: Jeep used to have a worrying amount white-blueish smoke come from the exhaust at idle, but since replacing the MAP sensor I have not had this issue.

Now here's something that I'm perplexed by... when I went to run an exhaust leak test, I noticed a surprising amount of positive pressure from the radiator with the radiator cap off... has anyone ever experienced this? If so, what could this mean? I can occasionally smell antifreeze while driving, but I haven't overheated since I had the radiator replaced, which was 5 days or so before replacing the motor mounts, manifold gasket and exhaust manifold. Previous radiator blew on me while driving; was forced to drive it for a minute or two into a nearby parking lot.

I'm not sure if I have a blown head gasket, or a warped head, or even a cracked block. I'm sure if the block is cracked, I'd be experiencing a lot more issues and probably wouldn't be able to drive. Any help is appreciated, this is my daily driver and only source of transportation. Thanks!

Have you tried swapping the original IAC back in? Sometimes the new box store parts are worse than the original worn mopar parts. Having disturbed things I would carefully inspect all the vacuum line for cracks or leaks. If the problem started right after the after doing the manifold gaskets, I would suspect a leak there or possibly the manifold isn't sitting right on the dowels.

Describe this "positive pressure" and how/when it is occurring ?

Have you performed a cooling system pressure test and a head gasket pressure test.


Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a " Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.

Sounds to me that you have either a blown head gasket or a cracked head that's allowing combustion gases to get into the antifreeze. Get the NAPA kit that checks for combustion gases in your antifreeze. If you find combustion gas in the antifreeze; it's time to pull the head and pressure test/magnaflux it for cracks.

Best regards.

CJR

OVERDUE UPDATE: Replaced the head gasket, and the head itself (which I probably didn't have to do, but I had the money to do so and wanted to keep Murphy from ****ing with me on this issue again). Idles great, runs great. Original gasket failed between cylinders 2 and 3, and the original head didn't seem to be cracked or warped. I already paid for the reman head at this point, so I just replaced it, along with the valve train (excluding lifters, as they were beautiful. Then again, so was the whole OEM drive train, but I wanted to give the ol' girl some love). Thank you to those of you who offered advice on what to do and where to go.