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Chug at start, "bouncing" tach when key turned to "on"...

KZOO3800

KZOO3800

New member
When I turn my key to "on" my tach does a strange "bounce" about once every second, ticking up nearly to 300rpm. Bounce, bounce, bounce. This is not present in any way when the car is running, but 8 out of 10 times I get a bouncy tach at "on". Could be CPS? Been this way for months. No other indication that the CPS is fubar. No stalls, just low idle.

On a cold start the engine starts beautifully, roars to life smoothly, RPMs slowly even-out over the first 30 seconds around 1000, temps always reach 195F and fluctuate normally, no overheat EVER. Drives great. After about 5 mins running after cold start RPM moves from 1000 down to about 600, low but stable. But, at warm start the RPMs literally snap right to about 800 then level off back to 600 after about 5 seconds of chug, chug, chug. This "snap" is bugging me because its always followed by 5 seconds of "chugging". This is mild, but noticeable every warm start. She idles pretty low at about 600RPM, seems a bit low to me, and its not the smoothest idle. At this point LIM gasket should have been changed... fearing compression leak or misfire.

I know it has a small exhaust leak, needs the MAF cleaned too, and I owe it a lot of general maintenance I cant afford at the moment. Would absolutely love to rebuild the heads and install new LIM gasket and valve cover gaskets. No fluid leaks in cooling, oil, trans, or brake systems. Any idea what could cause such a mild, yet consistent 5-second chug on warm start, or why my idle is slow, or why my tach likes to hop? Thanks.
 


See what your EGR is doing at idle. This strikes me as a vacuum leak, though, so check all your connections.

Also, try unplugging the MAF and seeing if it runs better.
 
Clean the MAF (it free assuming you have the proper cleaner for a MAF), then go from there.

After checking for vac leaks move onto tune-up. Plugs, wires, air and fuel filters. Most people forget about replacing the fuel filter for some reason.

Also, no need to "rebuild" the entire head. As long as it's not cracked/leaking then leave the heads alone and on the motor. HG issues with these motors are very rare. Just do the LIM gaskets and valve covers. You most likely do NOT have a compression leak. If you did then it would run like sh*t all the time, not just for a few seconds.
 
I have redone plugs, wires, air filter, oil change in last 4k miles, new thermostat (and boy did that help), coolant system flush (yeah I put dex back in, trying to isolate some problems unrelated to coolant). Im doing another oil change this weekend, and even have the new fuel filter ready to install.

What would you recommend as safe to clean the MAF? Also, are there any spots known for vacuum leaks I should check out first?
 
The man issues with vacuum lines tend to go from the fuel rail around the motor to the back...they typically dry rot from engine heat and age. Very common issue on the 97-03's. Auto part stores sell a MAF & Sensor cleaner for a few bucks you can try that...and if that fails you can visit...

Morad Parts Company - 3800 Motors, Transaxles, & Stock Parts and he can hook you up with a good MAF on the cheap.
 
CRC makes a MAF cleaner. Autozone or Oreillys should have it. If not they will have something similar. Either that or electronics parts cleaner for PC's and stuff would work.

Since I assume your car is not supercharged, I would say check the large vac. fitting that connects to the UIM at the back of the motor. It is a large connection w/ 2 lines coming to it. Other places include MAP/PCV area, UIM itself (around the edge where it seals to the LIM), throttle body area.

Just spray some carb/brake cleaner around all the places that could be vac leaks while the motor is running and see if you hit a spot that makes the tone/speed of the motor change. If so, then you found a leak.
 


No SES light, have not had it hooked up and tested in real time or otherwise though. PCV looks great, doesn't need replacement. Probably all O2 sensors should be checked out though, and the CAT. I'm trying slowly to understand how all these sensors and valves work together. EGR still confuses me, wikipedia states:

"With OBD II standards, vehicle manufacturers were required to test the EGR valve for functionality during driving. Some manufacturers use the MAP sensor to accomplish this. In these vehicles, they have a MAF sensor for their primary load sensor. The MAP sensor is then used for rationality checks and to test the EGR valve. The way they do this is during a deceleration of the vehicle when there is low absolute pressure in the intake manifold (i.e., a high vacuum present in the intake manifold relative to the outside air). During this low absolute pressure (i.e., high vacuum) the PCM will open the EGR valve and then monitor the MAP sensor's values. If the EGR is functioning properly, the manifold absolute pressure will increase as exhaust gases enter."

I can imagine that having MAP, MAF, EGR or O2 sensor problems may cause my chugging.
 
K, got some CRC MAF cleaner. Chick in front of me in line got chased by the cops for stealing Blue-Devil. I guess her pos Dodge needed it, because she got maybe 300 yards out of their parking lot before Kalamazoo cops swarmed her. Silly ho, stealing is for... hos.

Anyways, going to first unplug the MAF and see what happens, then I'm going to clean it and put a fresh air filter in. Then I'm going to check into EGR issues and O2 sensor issues. This seems an okay place to start.
 


Cleaned MAF, new oil filter, oil change, lucas stabilizer substituted for 20% oil capacity, new air filter. Results: no change. I spent about an hour under the car and discovered a moderate oil leak that is misting ALL OVER the back of the engine and soaking the oil pan, bottom of the trans, and front right portion of the frame and passenger side CV. I check the oil almost daily, and I've NEVER seen the level go down... not in 4 months of owning it. No idea where this is coming from. I check every fluid level at least once a week, same result: no change in fluid levels. Nothing shows up on cardboard placed under the engine overnight.

One thing is for sure, oil is misting out the backside of this engine somewhere, and covering everything it touches. Found quarter sized hole in exhaust (dunno what its called, but its the exhaust component in the middle of the underside of the car near the downstream O2 sensor). So, idle for this car absolutely sucks, but on the highway and city driving though, its balls to walls, albeit slight loss of power from 159K miles the previous owner and his sister put on the car with little or no maintenance AT ALL! Leave it to a Lebanese guy to run a 3.8 Series II into the ground. Previous owners did not know the phrase "routine maintenance"....

I'm not kidding, before I bought the car I asked him if he changed his fuel filter or oil, spark plugs, wires, air filter... and as I popped the hood and pointed all this s*it out to him he gave me a blank look, as if the car was supposed to have performed its own maintenance for 4 years. He didn't even know where the oil or trans dipstick was. Never took the engine cover off... At $1400 I cant complain, but still, some people should be forced to ride bikes everywhere until they can at least do an oil change... Idiots...
 
im having same problems. but my engine light is on.
the hole in your exhaust is the resonator. i have about 10 holes in mine i need to replace.
im going to clean my MAF and see if it helps but im not sure what else to check for
 
Well incidentally by replacing the missing #15 micro relay in my engine fuse box I have eliminated my trac off/check tire press/low trac light that usually came on after about 30 seconds of driving, and stayed on until restarting. Still have the bouncing tach at "on". I threw my ABS fuse back in and have yet to drive it, but i'm suspecting the malfunctions I had with ABS that caused me to remove the ABS fuse altogether may not have been mechanical, but due to the absence of the relay. I will post later today about the results.
 
for the ABS: On each wheel, next to the caliper there is a connector that goes to the wheel speed sensor. The wires break at the connector- no wheel speed input. also gives a trac off if it's in the front.
 


My car used to chug also at idle bouncing between 600 and 800 rpm got it fixed by putting the small o-ring and spring that was missing on the PCV for some reason previous owner of the car took them out. and when I bought the car I didn't know it came with those missing parts and only swapped the PCV out for the new one. It was until a few days ago I asked here on the forum and they pointed me in the right direction. What I did notice on mine was that with the engine on I took the oil cap off and I could hear the engine sucking in air, check if yours does the same thing and also if the 2 o-rings (one small goes on the PVC and one large on the twist cap) and spring are there.
 
I've checked my PCV and MAP sensor. I couldn't find a problem visually or otherwise, both the o-rings are in place so I cleaned the whole area ad oiled the o-rings with new engine oil.

I'm expecting improvements after I replace the UIM and the LIM gaskets with the valve cover gaskets. May even do head gaskets. I've got moisture around every gasket-location on the engine. Moisture all over the LIM from the UIM gasket area, moisture all over the head area, and moisture directly below the valve cover gaskets. Fluid levels remain stable so by no means am I just pissing away oil or coolant but I can't really justify posting more until I actually bite the bullet and have all necessary gaskets replaced.
 
Head gaskets rarely blow on these motors, so I'd leave that off the list. Besides, then you will have to buy all new head bolts as the factory ones are TTY and are not reuseable.
 
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