Hey guys, I need your help. I've been having this problem since I bought the car a year ago . When the engine warms up, it's starts chugging like a steam train! You can feel the vibration, but still idles at around 700 rpm. It's only noticable when the car is not moving, either in park or stopped in drive, not both . And it accelerates fine with no misfire or CEL codes. At a stoplight easing off the brakes, it inches forward in a chugging motion. The exhaust also chugs and not smooth and continuous. What the hell is going on? I haven't found a description or an answer anywhere in the web.

I've replaced to a new Mopar CPK, Duralast distributor, Duralast spark wires, Bosch spark plugs, Bosch O2 sensors, MAP sensor, IAC, muffler and tail pipe, and cleaned throttle body thoroughly . I've checked out the fuel pressure, cylinder compression, flex plate, and professionally checked the transmission. Everything is normal.

I've been scratching my head. I might to switch to NGK O2 sensors, champion copper plugs, new fuel injectors, replace the PCM, front pipe and cat converter in case they're clogged. This is all pretty expensive and would love to hear any ideas . Thanks!

99 Cherokee XJ 4.0 6 cylinder 4wd

 

Forget about replacing the PCM, that almost assuredly is not in play here.

Next thing I would do instead of throwing more parts would be to take it to a shop where they can analyze your exhaust and back pressure to see if you might have an exhaust issue as you mentioned. Also be sure that your CCV (crankcase ventilation system) is working properly and without blockage.

I applaud your tenacity and I'm absolutely all for DIY to a point. That point is when parts start getting installed without testing that shows them to be bad. Sometimes the best money you can spend is on a quality diagnosis at a trusted shop that employs ASE technicians. As the symptoms sound repeatable, I'd be surprised if a good tech can't figure it out within an hour or so.

Good luck and keep us updated!

 

Disconnect the exhaust down pipe if your 99- and run it to check for clogged cat.

 

Taking it in for diagnostics isn’t a bad idea, but if it were mine...

To me the symptoms indicate a cylinder (or two?) not firing for some reason, at idle. New plugs/wires all that jazz would indicate it’s probably not a lack of spark, and could be a lack of fuel at idle.

If it is consistently not firing in one cylinder you can find out which one by pulling spark plug wires, one by one and replacing them while the engine is idling. Pull a wire on a cylinder that is firing well and it will idle noticeably worse, or stall. If they all have a noticeable effect on how the engine runs then they are likely all firing correctly. Pull a wire on a cylinder that is not firing well and you’ll notice no difference. If this is the case at least you’ve narrowed the issue down to a single cylinder (or two).

Assuming there is no reason to think the spark plugs are malfuctioning as all those parts are new and installed correctly, I would then focus on the injector(s). Check electrical connections, o rings/seals. Lastly I’d back flush them with carb cleaner. There is a method to this requiring bench actuating them with a 12v battery and flushing carb cleaner through, backwards.

If you did want to fire the parts cannon again, I’d suggest packing it with injectors and a fuel filter. There’s a guy in vendors selling refurbished four hole injectors at a reasonable price. May as well upgrade.

 

That's how mine runs and it's a misfire. If it was a bad plug it would likely run like crap throughout the rpm range not just idling. Try doing a leakdown test. Mine has great compression but failed the leakdown test in cylinder 1. Also have you verified the check engine light works?

 

Thanks for your suggestion. You're probably right, I should bring it over to get it diagnosed. There are probably a few more tests I want to perform before I do . Last few guys I went to said they didn't know what was going on and that it was normal for an old car. Boooooo, that's lazy.

 

It is firing on all cylinders. I did used to get a code for misfire on cylinder 1, but that went away. I will try your test on pulling the spark plugs. The weird part is that it only had this problem in park and neutral, and not in drive with the brakes on. Sometimes it's only in drive with the brakes on and not in park or neutral. If it was a misfire, wouldn't it happen regardless? The injectors we're next on my list to check out. It seems like a pain in the *** and not looking forward to it .

 

The plugs are good and getting spark on all of them. But I will try pulling out the plus one by one and see if it's changes the way it idles. I will try the leakdown test but I need to get my hands on the guage. You have any recommendations for a guage or some how to videos I can watch? The check engine is definitely working .I changed out my CPS a few days ago after I got a CEL .

 

Update:
I already ordered NTK O2 sensors, upstream and downstream. It seems to run a little smoother but the chugging is still there. Keep in mind that it doesn't chug while warming up. It only happens after i reach normal operating temperature.

I changed the oil. Took a little sample to send to Blackstone labs. When I removed the oil filter, I decided to pour the remaining oil in it into a clean clear container, just to see what kind of particles I find. I found some very small black granules, almost like soot, but very little. In the sun, I also noticed that there was very very tiny gold dust floating around. It wasn't barely noticable, but it sparkled in the sun. This is bad news right?

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https://www.google.com/search?client...6eAfIKON1qM%3D

 

Likely you could rent the test gauge from autozone or someplace similar. But do you have or have access to an air compressor?

 

I just checked with my AutoZone and they don't have one. So I might have to purchase it. I do have an air compressor. It seems like a pretty easy test.

On my last update I mentioned noticing some metal dust in the oil, as well as some shavings. Since the color is gold, it could be a spun bearing, or bad lifters which also might be causing my engine to chug? What do you think of my theory?

Update:
No tests today. Went wheeling instead. Noticed a drop to almost zero on the oil pressure guage and a check guages light. Everything went back to normal after a hike. I'm going to be replacing the oil sending unit. On the bright side, pushed my wheeling capabilities. I came out on top.

 

SOLVED???
i might be optimistic but ever since I've changed the oil pressure sending unit, my chugging has stopped. The chugging was a 99% repeatable condition after 10 min of driving. I've been driving for two hours on and off and it's gone! Sometimes when I throw it into park, I can feel the engine building up to a chug but it corrects itself.

This is perplexing but I'll take it. It makes me think that it's an issue somehow related to the oil .I'm going to do an oil flush for good measure. I've only done one oil change since I bought it. I'm almost certain that the previous owner neglected the car of any maintenance.

I'll keep you guys updated.

 

I can’t see how the two could be related. I believe the only function of the oil pressure sending unit is to allow your oil pressure gauge to show a reading.

I’d be interested to hear a viable explanation.

 

Well I will keep you updated on whether or not my jeep continues to run smoothly.

When I took off the old sending unit, I noticed that there was oil all over the prongs and gathering in the connectors as well. I cleaned out the female as best as possible and installed the new one. I did not perform a hard reset on the PCM.

I looked up a diagram that shows the sending unit directly hooking up to the PCM and then to the gauges on the dash. Do you think that all that oil was somehow sending bad signals to the PCM?

*Fingers Crossed*