3.4L DOHC Running Rough

[Danspeed1]

I have a 1995 Chevy Monte Carlo Z34 3.4L DOHC. The car has 231,000 miles on it and had ran great up until now. I got in the car and started it up the other day and it began running very rough. It was dead cold when it started running rough and that did not change after it warmed up. The car is "chugging." I put two cans of sea foam into the tank and filled it with 93 Octane fuel. No change. I pulled the MAF sensor wires, ran worse. Pulled the EGR Wires, no difference. Pulled the Idle Air Wires, no difference. When I began pulling wires for the injectors the car ran even worse, until I got to the last one. Its hard to tell but it didn't seem like it was running any differently which leads me to believe an injector went bad??? Does anyone have any idea of tests I could do or what it could be? There is no MIL light. I have a set of new FORD 19Lb Injectors that I was going to use for another project; will they fit, are they safe to use, will the car run alright or is it a waste of time?

Dan

[Schmieder]

I have a 1995 Chevy Monte Carlo Z34 3.4L DOHC. The car has 231,000 miles on it and had ran great up until now. I got in the car and started it up the other day and it began running very rough. It was dead cold when it started running rough and that did not change after it warmed up. The car is "chugging." I put two cans of sea foam into the tank and filled it with 93 Octane fuel. No change. I pulled the MAF sensor wires, ran worse. Pulled the EGR Wires, no difference. Pulled the Idle Air Wires, no difference. When I began pulling wires for the injectors the car ran even worse, until I got to the last one. Its hard to tell but it didn't seem like it was running any differently which leads me to believe an injector went bad??? Does anyone have any idea of tests I could do or what it could be? There is no MIL light. I have a set of new FORD 19Lb Injectors that I was going to use for another project; will they fit, are they safe to use, will the car run alright or is it a waste of time?

Dan

Doesn't mean an injector is bad, could be but disabling an injector on a cylinder that is already misfiring will still misfire. At least you have it located down to one cylinder.

Did you check the spark plug? Do that while the engine is cool. Never remove a plug from a warm block, could strip the threads. The spark plug analysis will help diagnose or rule out whatever is wrong.

Since you first noticed the issue during cold start, I suspect the problem occured during heat soak from last time it was operated. Can you clue me in to what kind of conditions the engine was in during the last known good operation. i.e. was it a long trip or short few blocks? Did it reach operating temp or is the car overheating slightly? Was it driven aggressively or soft pedal?

I would wait on the injectors for now. No need to complicate the diagnosis of what is wrong.

Bad spark plug, clogged/dead fuel injector, bad fuel injector wire, spark plug cable is broken.......could be compression related. Get a pressure gauge that fits your spark plug thread to test cylinder leak down pressure.

Since the problem is isolated to one cylinder we can generally rule out global systems, such as fuel pump, alternator as these items would effect all pistons universally. I doubt it is the coil as that would effect two cylinders. But the coil should be tested if all else fails.

While the car is running, have a friend rev it up and see if there is a spark arcing off the spark plug cable to the cylinder that is misfiring. If the cable is bad the charge will arc out and you can see it as the engine is rev'ed up.

Take a picture of the spark plug or read it yourself to see what it tells you. This can tell us a lot, like running too lean or rich or if there is mechanical damage, etc.

[pocket-rocket]

I agree with Schmieder on the compression test. Also, I've heard of DOHC injectors sticking and rapping on them with the handle of a screwdriver will get them "un-stuck" when the engine is running. Hopefully the bad injector is one of the front 3 since getting to the rear ones would be a major pain. And, IIRC, those Ford injectors are going to be too small to use on the DOHC, especially without tuning.

[Danspeed1]

I pulled the injector I thought was bad today and gave it a bench test and cleaning. It sprays very well so its not the injector. I would jump to an 02 sensor but this problem starts from dead cold to warm so I can rule that out. Pulled the MAF wires it made it worse, same with the EGR and Idle Air controller. I am going to try throwing a set of plugs at it and see if it makes a difference.

DG

[bszopi]

How about the Air Intake Temp sensor? If the PCM thinks it is super cold, or really hot out, it will affect the fueling.

[pocket-rocket]

How about the Air Intake Temp sensor? If the PCM thinks it is super cold, or really hot out, it will affect the fueling.

That would have an effect on all cylinders though, not just 1.

No compression test yet, Dan?

[Danspeed1]

Well, I ruled out the Injectors and I just installed new spark plugs. Although it needed plugs this did not fix the issue. In fact the issue seems more apparent now.

Now the engine is running smooth, but then missing every couple of seconds. It will run smooth, then miss, run smooth for another few seconds then miss.

When the car is warm something very interesting is happening. I have an intermittent check engine light. When the light comes on the car loses most its power, and starts to spudder and chug really bad, but then the light goes off and the idle will clear up. I have OBD 1.5; this has been plaguing me for years because I never have a way to check the codes!

I don't think its compression related as I have good power about 3000 RPM.

Coils are from autozone and are a year old.

DG

[robertisaar]

the 94-95 LQ1 doesn't use the paperclip trick for flashing the SES light, however there are scanners that will work, and serial/USB cables for hooking up to a laptop work as well.

[Danspeed1]

Alright,

The problem was actually partially resolved. A new set of plugs, sea foam, and fresh gas fixed it for the most part. However I still have a situation I feel is being caused by one of the sensors. It primarily occurs after the car is warm, however it may happen when its cold and i am just not noticing it. its most noticeable at stoplights. I will be sitting there, and the car will be idling just fine. Then the check engine will turn on, and the car will immediately begin to run rough. If I wait, after a minute or so it will go off and the car will run fine again.... or I can give it a little rev and just as soon as it get s over 2500 RPM the light goes off and the idle clears up. It has to be a sensor... just not sure which one.

DG

[robertisaar]

be a lot cheaper to scan for codes than throw parts at it, since the LQ1 stores history codes. get the SES to come on, shut the engine off with the fault still active, wait a few minutes, restart and go on if necessary. then find someone with a scanner within 50 ignition cycles and you'll find out what code is getting set...