Rough idle

#1

A 2003 Chevy Malibu 116,000 miles. Check engine light on. Major tune up done. Fuel injectors replaced. Compression at 125. Rough idle and engine light is still on. Mechanic said get a second opinion. $1400.00 later. Now what???

#2

could be a vac leak some were. try working with that.

#3

Now, use that built-in diagnostic system…the DTC codes that are indicated by the check engine light. If your mechanic can’t scan and retrieve the DTCs; then, ask an auto parts store to scan the engine computer for you and brings those codes here for advice.

#4

That compression seems really low.

Certainly start w/ the codes as hellokit suggested. But be worried about the intake manifold and have it checked for leaks. The Malibus are known for chronic problems with that.

#5

They should have stopped at the 125 PSI part of this before replacing injectors and whatnot.
Surely the shop is not saying that figure is acceptable?
A good engine should provide readings in the 175 PSI and up range.

Without knowing if you bought the car new or used and any history behind it, it could more than likely be a ring problem if the 125 is present in all cylinders.
This means either lack of regular oil changes in the past, overheating engine, or possibly a leaking intake gasket which is allowing coolant to wash the cylinder walls down and which in turn wears out the rings prematurely.

When they tested the compression at 125 they should have followed this up with a “wet” compression test, which means repeating the process with a squirt of oil in each cylinder. If the reading takes a fairly good jump upwards then the rings are at fault.

#6

The car is 2 owners. I have had it since 19,000 miles and for 3 years. I average 33,000 miles a year and oil is changed every 5,000 to 7,000 miles.

#7

As ok notes - given 125 compression and regular oil changes I will say again to look at an intake leak - the kind that dumps coolant into the cylinders.

#8

Okay, here’s more info. Garage said. checked for vac leak and checked intake manifold. The computer code is showing a problem in cyl.#6/PO306. they did NOT do the “wet” compression test because the car was not showing symptoms toward rings.
thanks so much

#9

You need to find another shop.
A 125 PSI is too low and any potential intake manifold leak or misfiring cylinder has nothing at all to do with the the compression reading.

With a 125 PSI reading a wet test is a must; not an option. At 125 it’s either rings or valves or both. If a wet test is done and the readings jump to 150 PSI or whatever then the rings are at fault.

The rings should NOT be worn out at 116k miles but as I stated; if the engine has overheated in the past or an intake manifold leak was oozing coolant into the combustion chambers the coolant can prematurely wear the rings out.

#10

What is a 125 PSI? I am confused…I stated in my original question that the compression was 125. So what is this 125 PSI?
One last thing. The car runs GREAT! Only problem, the rough idle. Also, I don’t know if this matters but when I put the car in park or neutral the rough idle goes away.

#11

PSI is pounds per square inch. It is a standard way of measuring air pressure.

#12

Pleasedodgevan is correct. Sometimes running great can be a subjective opinion and what can happen is that one becomes acclimated to an engine that is tapering off in performance. The same thing occurs with shock aborbers/struts; one becomes accustomed to a harsher ride and does not notice it.

Now, the compression is 125. Does this mean on every single cylinder or only one?
If you have a 6 cylinder for example and the compresssion is say 150 on 5 cylinders and 125 on the remaining one then yes, you can have a rough idle because of that.
The idle is controlled the Idle Air Control Valve which is controlled by the ECM. Putting it in gear provides more input to the IAC valve and this could be overriding a problem.

The only thing I can add is that a 125 PSI is too low IF this reading is legitimate. It’s also possible to have a leaking compression tester (Schrader valve problem similar to a tire valve stem) and maybe the readings are inaccurate.
JMHO, but anytime I’ve run into an apparent engine problem with compression I always go back and retest it with another gauge; and sometimes twice more.
Engine work is expensive and guessing should not be part of the diagnosis.

If possible, you might post the readings as given for each cylinder. They should all be way higher than 125 and they should all be within 10% or so of each other.
Hope some of that helps.

#13

Nice reply OK. I often find that responses seem to assume that the OP understands everything involved with the problem and so will understand everything about a cryptic reply. If the OP understood all of those things, s/he probably wouldn’t be posting. You took him/her through the issue in a friendly, non-judgmental way and laid out the considerations in plain language. That’s when a post becomes useful.

#14

Yes, I agree. You all have been so helpful. The garage did NOT check ALL the cylinders only #6 because the computer said that’s where the problem was. Please believe me I am taking in all your wonderful advice. I am going to another garage. I was ONLY trying to give the first garage the benefit of the doubt and I did NOT want to misjudge them. When I do find out what the problem is, I’ll let you all know. P.S. I now know why woman shouldn’t go to garages by themselves. lol