Questionplugs look good... no vacuum leaks that i can find? no visible cracks around the o2 sensor. let me know what to do next... thanks again, mark rossiter
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Followup To
Question -
hello, i have an 89 gmc 2wd s-15 2.5 cyl that is missing (and getting worse) and i can't figure it out. in the last week i've changed the plugs, cap, rotor, wires, and fuel filter. about 6 months ago i changed the o2 sensor. when i changed the o2 sensor it was running rough and the check engine light came on. i checked the codes, changed it, and it has ran great since. this time there is no engine light coming on and it is missing differently (harder i guess). sometimes it smooths out for a few seconds with acceleration. i don't think it's the o2 sensor again but i don't know? i took it out and cleaned it with carburetor cleaner and there was no difference. i was told to try that. i know it's starting to get expensive. i checked a manual and it seems there's a lot of other sensors it could be. i don't know how to figure it out without any engine light codes. i hate to buy parts and use the trial and error method... any help would be greatly appreciated. one other thing to note is that this past summer i noticed it was hard to start when hot. i read that's the sign of a bad ignition/control module in the distributor? the guys at napa said that wouldn't be the problem... i don't know? thanks, please help!
mark rossiter
Answer -
Gm had some problems with their design of the 2.5L engine of which some were:
1. The intake manifold was not manufactured correctly as it was not delivering an equal amount of fuel and air into the combustion chambers.
2. There have been numerous cases of exhaust manifolds developing a crack in the manifold and when this happens atmospheric air is sucked into the crack and the air/fuel mixture passes the O2 sensor of which reads a LEAN fuel mixture which in turn causes the computer to turn the injectors on and off more quickly thus over loading the combustion chambers with the incorrect fuel to air ratio which causes a MISS in the engine.
There First trial to even out the air/fuel ratio to the combustion chambers was to install a special plate under the fuel injector body assembly to divert the fuel mixture EVENLY to each combustion chamber. Next was to redesign the intake manifold and to replace the ECM (computer) chip of which also was a part of the alleged fix process.
Pull the spark plugs and see if they all have the same colorization on the electrodes and let me know.
Also be aware that ANY type of vacuum leak will also cause this type of miss.
I agree on the ignition module and or control module as being the fault.
Let me know what you find.
autohelp
AnswerOK this would be your next set of procedures.
1. Perform a compression test on all cylinders.
2. Perform a cylinder balance test, whereas you install a tachometer to the vehicle to physically see the engine RPM'S and short out one spark plug wire at a time to see which cylinder reduces the RPM'S significantly more as compared to the other cylinders.
Run the tests and let me know the results.
NOTE: Double check back over your previous work on the vehicle as you may have a hair line crack of the porcelin (white area) of one of the spark plugs that you installed. The crack can sometimes be very difficult to see.
Have you noticed if there is any whitish/gray residue on the underneath of the oil filler cap or a brown color to the engine coolant ?.
Let me know what you find.
autohelp
