JohnD

Several of us have had problems with setting a code for random misfire and as far as I know, nobody has actually found a definitive cure. We have tried all things ingition related and nothing seems to help. Mine is worst when the ac is on at idle. It will sit there and idle rough until the SES light finally comes on. The hotter it is, the worse the problem (load on ac?). Anyway, yesterday, I replaced my PCV valve and the problem is noticably better. Could it be that easy? I always clean PCV valves and verify that they aren't stuck so I've never been very big on just replacing them but I did this time and the problem is noticably better. Anybody else try this?

slopokrodrigez

I noticed you said better. But does the misfire code still set? Have you searched for vacuum leaks? Is it setting for one plug more than the others?

Last question, you say it loads up worse with the AC on and you have AC mods listed in your signature. What type of mods did you do to the AC?

JohnD

It still idles a little rough with the ac on but just not as bad. So far it hasn't set a code but the outside temp hasn't hit 100 deg. yet either.

The misfire is entirely random and does not point to any one plug or cylinder. I replaced the plugs just to see with no change.

The ac mods have to do with the fan system. I added a switch and a thermal switch. The thermal switch is in the head where the temp gauge used to go and the gauge now comes off of a dual unit in the water pump. One setting of the switch is normal, one runs the fans on low all the time and one setting allows the thermal switch in the head to control the fans which means it tries to keep the head temp under 207 deg. None of these has anything to do with the actual ac system or the mis which developed early last summer.

There has been a lot of discussion about this random misfire on these cars and nobody seems to have a solution. I was hoping that I might have stumbled onto one. The part that bothers me is that it never happens with the ac off. It is almost like the compressor is dragging it down. How can that be? Pressures are normal and unit cools fine.

Oh yeah, if there are any vacuum leaks, I haven't found them. With a fuel injected car, a vacuum leak usually results in a fast idle and I don't have that problem at all.

guywithaZ

A guy I knew had a bad PCV valve in his TA...ran so bad they thought it needed a new engine...changed that and voila...

slopokrodrigez

Your the second person I talked to on this board with misfires when the AC was on only. I believe it has to do with a (for lack of a better term) jerky motion to the serpentine belt. Remember misfires are monitored by the CKP which uses the crank at the timing cover. Something you could do to see if the rotational speed of the AC compressor is changing by putting a white mark on the face of the clutch and use something like a timing light that blinks at a consistent rate to see if the position of the white mark changes. You may have to do it with help monitoring the misfire counters at the same time. This might correlate a relationship to the AC input shaft with misfires???????

One other thing. I have encountered several vehicles that "ate" alternators only to find out the tensioner for the serpentine belt was not changed. Once the tensioner was changed, the problem of "eating" alternators was gone. We assumed the tensioner was binding and creating a jerking motion to the belt. This inturn loads and unloads the input to the alternator. We have found both mechanical failure to the bearings and electrical failure to alternators as a result of bad tensioners. We recommend changing the tensioner any time the belt is changed and doing both when ever an alternator is changed

My point is, perhaps the tensioner were your problem but it doesn't have enough drag or jumpiness to effect the crank until the crank is put under more load by the AC. Doesn't seem as though it would or I should say it would seem that the problem would be there all the time if the tensioner were the problem.

Perhaps you could contact a reputable dealer and ask if there are any tech bulletins regarding the type problem you are having.

Tom Piper

I saw a PCV problem on a 5.0 Liter Ford engine that caused the #8 cylinder to consistently oil-foul the spark plug at any highway speed in about 20 miles.
The #8 cylinder would pump so much oil that it would lower the octane rating of the fuel-mixture and cause severe detonation.

The cure? I replaced the rubber grommet that the PCV valve sits in -- ran like a new engine. This rubber grommet became so hardened that it no longer sealed around the PCV valve.

At highway speeds, with the engine's downward slant toward the rear, the oil pump would pump enough oil to the rocker arms that it would form a pool of oil around the last valves on each side of the engine (unlike a Chevy, the last valve on the driver's side (#8) of a 5.0 Ford engine is an intake valve). And, the crankcase pressure would force the oil past the intake valve guide -- even though the valve guide was not worn.

Replaced the $3.00 grommet, and all the problems went away -- like having a new engine.

I have a new-found respect for crankcase ventilation after that.



Tom Piper

JohnD

The serpentine belt is something that had not crossed my mind. I have a new belt and a new tensioner (which is already starting to squeak-teach me to buy an aftermarket one with a plastic roller). I'll try the timing light trick and see if anything shows up. Thanks for the tip. A new grommet for the PCV wouldn't hurt anything either.

jpack24

Well this happened to me also. At first I thought it was the opti. But this problem only happened when the Water temp climbed. You might want to try this. It worked for me and havent had a problem since. Thanks to shoebox and his informative website

http://shbox.com/1/4th_gen_tech2.html#opti_test

Scroll down to Icm cooling mod. Hope this works out for you. Let us know.

JohnD

That's an interesting idea and one that I hadn't thought of. I always assumed that the designers of the engine bolted the ICM to the head knowing that it would be safe as long the temperature did not exceed the head temperature knowing that, under certain conditions, the engine compartment temperature might exceed the head temp. I think I'll try this and if the problem resolves, to me this says "replace module." My experience with the older GM ingition modules is that they usually fail around 85K and I wasn't thinking about this as a problem as I am only at 68K at the moment.

slopokrodrigez

Interesting theory. There are 2 things you can do to test for this type of failure. One, does the problem occur as soon as the AC comes on even if the engine is dead cold?

Two, you can cool the ICM quickly and easily with freeze spray available at any good electronics shop. If the problem only occured from heat, cooling it down should return it to good operation.

JohnD

At this point I think I can safely say the the PCV valve did not resolve the problem at all, even if the car's idle was a little better. On the way home last night I sat at a long light and left the AC on and sure enough the check engine light came on. Scan with autotap at home revealed the usual random mis code. I squirted carb cleaner around the pcv and vacuum lines just to verify that there was no vacuum leak to start. The car idles smoothly until you turn on the ac. The idle speed doesn't drop but there is a noticeable roughness and you can see the misfire counts start piling up slowly. The hotter that the outside air temp is (and consequently, the higher load on the compressor) the worse it is. I can speed the idle up to the high 900's and the mis counts go completely away. Other than this the car runs perfectly. I swear, I think it is the AC, somehow. I don't have much else to blame-I didn't get a misfire code all winter.

Tom Piper

Just as a guess, since it only happens with the AC on, I would check all grounds -- engine, frame, and especially to the computer (ECM/PCM).

The only other thing that comes to mind is a fuel-mixture that is border-line lean -- the extra load of the compressor is enough to cause it to miss.


Tom Piper

pillagenburn

I'm having a very similar problem only that when i turn on the car it misfires from the get-go. Had codes scanned and it said lean on (i believe) bank 2. I replaced the MAF because i have a warranty on it and i wanted to be sure that wasn't the problem. My car has 111k miles on it, i'm going to replace the PCV grommet but i don't know how to replace the PCV valve though i'm sure its not difficult.

Basically my car runs like a cross between a fried plug wire and a massive, gaping intake leak/unplugged MAF

valter

I am having similar problem. I think AC just adds a load to engine, and PCM make adjustments on "AC On signal" and different MAP sensor output. I noticed, that mine LT1 runs rich with AC on. I have no vacuum leaks, MAP, TPS, IAT all new. Cooling IAT not helping much too. Probably only full scan results can help. Temporarily I just increased RPM and misfires count thresholds in PCM, so P0300 are not appearing any more.

pillagenburn

if/when i put a new engine in, im seriously just going to yank my entire A/C system out ... i NEVER use it (MAYBE once the entire year - even though i live in Georgia)...... weight reduction, vastly more space in the engine bay - just hope it doesn't cause any problems...

but for now, i have NO idea what could be causing this misfire... i'm going to yank the belt tonight or tomorrow and see if any of the pulleys have locked up, that might give me some insight