I posted earlier about my car setting a code which is for random misfire. I had just put it off to a bad tank of gas when, two tanks later, it happened again. I did some monitoring on the Autotap and here's what I found: The car can idle and when I monitor the cylinder misfires, there aren't any, or at least a very seldom one. When I turn on the air conditioner at idle, misfires start happening over all the cylinders, slightly more on some than others but mostly random. The count will continue to climb and set a misfire code within a few minutes of idling. Has anyone ever experienced anything like this before? I'm mystified. Seeing as how in the winter in Texas you only run AC in the afternoons as opposed to day and night the rest of the year, this constitutes a problem. I'm still proceeding with the thinking that it is somehow ignition related. I did a plug change with no change, wires are next, I guess.

 

wires should be first... I am having a random missfire almost positively because of a burnt plug wire. I noticed it when I was putting the wires back in that one of them was touching the header... I have not noticed any correlation w/ the A/C however, which could mean you have a different problem. Wires are here, if I notice no change I will repost but I'm almost positive that is my problem.

in 6th gear going about 50 or so, do you get a bad vibration back and forth? (this is another one of my symptoms)

 

My first question John is what happens when you have the engine reved to about 2K in park with the A/C off?

Have you ever removed the CKP to see if it's contaminated in anyway?

As you watch the counts go up, the SES lights cause of a DTC300 right?

Do you see any light show with the hood up and the A/C on in the dark?

What does it do when you drive it without the A/C on?

Does the idle speed change when you switch on the A/C idle in park?

 

CKP?
yep
nope
nothing, runs fine
not much if any, I've wondered about that
on a side note, I just realized that my belt idler bearing is failing...hmmmm.

 

I've been thinking about this problem for a while. After reading all 800 some posts of the thread about this a while back and seeing how so many different probs can cause the exact same symptoms I have started to believe that there is just some inherent instability in the LT1 electrical system. One wire, or one sensor goes bad and you get P0300, multiple random cylinder misfires, or a cascade of sensor failures subsequently take place. I have misfires on every cylinder but one. I inspected my fire sleeves and they looked perfect so I went elsewhere looking for a problem and replaced a few things without improvement so I eventually went back to the wires and found that under the perfect fire sleeves 2 wires were burnt up. Now why should I be missing on every cylinder and ONLY in closed loop if its only 2 wires are bad? I dont know, but I have heard on this board several times where someone has experienced RANDOM AND SEVERAL simultaneous cylinder misfire with only two or even one bad wire where according to the computer the bad wires are not necessarily the cylinders that are misfiring!? I am just finishing up putting new wires on and I will tell you if it the two wires fix the 7 misfires, it makes no sense, but I think it will, seen it before (but only on LT1's). Just a thought.

 

CKP is the crank position sensor. The PCM watches the signal of the CKP to determine when the next signal should arrive. If it's out of sync it triggers a misfire. If this sensor is contaminated since it is hall effect, it can read incorrectly. However, you would expect it to read incorrectly all the time. Not just when the A/C is on. But on the other hand it is worth checking since it's easy.

The belt tensioner you mentioned is critical for long alternator life. We have seen where a bearing going bad in the tensioner will sometimes cause the belt to jerk and wear out the bearing on the alternator. Since the belt is connected to the front of the crank and drives the A/C compressor, I would say to replace it right away. You never know if the tensioner is jerking the belt and loading and unloading the crank to throw off the CKP and cause the code. Would be interesting if that is the case.

One thing that we see alot of on other GM vehicles is the power steering pressure switch is defective and the PCM does not raise the idle speed of the engine when the steering is loaded like when turning. These vehicles will have a problem when turning a corner. That is why I asked you about the idle speed changing when the A/C is engaged. If the PCM does not raise the idle speed, there could be some problem with engine lugging with the A/C on.

Please keep us informed what you find. This is a very interesting and critical issue for perhaps many people.

One last thing John, you didn't answer the question I had about revving the engine to 2K with the A/C off. What happens?

Dave

 

About the rev to 2K with no ac-Works just fine. Incidently, sometimes at lights I will try to idle the engine up to 1200 or so with the ac on to try and smooth it out. I've noticed that it runs rough even at 1100-1200 with the ac on. The reason why I didn't jump on wires in the very beginning is because the car runs so well under heavy acceleration, with or without ac. My past experience has perhaps led me down the wrong path. I'm buying new wires as soon as I can afford it and I'll set everybody know what happens then.

 

Change the belt tensioner since you know it's bad.

 

Don't worry, that's the game plan.

 

Smack us all if you change the belt tensioner and the problem goes away. But that makes sense if the problem only occurs when the A/C is on. The crank pos sensor CKP is on the end of the crank and watches for changes in rotational speed of the crank. Since all of the cylinders are fine until you engage the A/C then ALL the cylinders show missfires, I'de say you have something there.

Could also be the A/C compressor if what we think is happening.

Please let us know.
Dave

 

Ok, so I replaced the tensioner and the squeal went away but nothing else changed so that isn't the mis problem. Here is what else I found. With the AC off, no mis. With the ac on and temp less than 70 deg. F, no mis. The hotter it gets, the worse the mis gets at idle and the rougher the idle. Note that this problem completely goes away when I cut the ac off or run the motor up to 1200 rpm or above. Even though the ac is still cold (sensor is reading 48 deg F. at the evaporator) clearly the compressor is laboring and it is making some noise. The idle speed BTW is 830 or thereabouts and doesn't change with the ac on. The pressure sensor voltage gets up the 3 v.-does anybody know what head pressure this indicates? My factory manual doesn't show. This looks for all the world like my ac compressor is dragging down the engine to the point that it is creating the random mis. Bizarre but i can't think of any explaination, given the facts. Ideas?

 

Is the PCM detecting the A/C is on when you scan it? Your problem may be similar to the steering problem I mentioned where the engine doesn't see the load and doesn't compensate by raising the idle. You'll want to look at the AC Status not the request.

As far as pressure goes, the sensor is 0 to 5 volts for 15 to 452 psi. Assuming it's linear, 3 volts should be around 277psi. If my math is correct.

Also I thought of something, does your primary fan come on when you switch on the A/C?

I wonder if you could remove the belt but leave everything else connected and see what happens to the counts when the A/C is turned on? If that could be done, it may answer a lot of questions.

BTW, did you ever pull out and check the CKP for contamination? It would not hurt to do so.

Dave