08 Corvette. When I run my fan using the VCM fan control I get multiple misfires and sputtering. As I increase fan speed misfire gets worse. I'm running in Open Loop. In CL I get misfires even with fan off. With fan off OL engine runs fine with no misfire. How can this happen?
I think deanm11 and I have lemon 08 Corvettes!
Anybody? Could my mass air be the culprit? What do the fans do that could possibly cause a misfire?
Back from the dead. Since the injectors have tables that control with voltage would it be possible the fans running wide open could cause lower voltage and leaner idle AFR and eventual lean misfire? Anyone?
Interesting because both of my 02 narrow band values go down to the double digit values when the fan is running. I feel it is going lean but why? Could I modify the injector control to compensate?Originally Posted by Nmbr1GMfan
I took the car to the dealer that said my headers and intake would not be a problem that they would diagnose the problem anyway. They kept the car for 4 days running all kinds of test only to conclude it was my aftermarket intake. I had the problem before the intake on so I know they just couldn't find the problem and had to blame it on something.
I went out this morning and logged voltage with the fans on and off. There was very little voltage difference. My NB values changed as well I noticed, Im sure someone with more knowledge on the subject will chime in. Just wondering is your miss only at low speeds then clears up at cruise? After a WOT run is it any better? does heat (since fans are on) seem to be when the problem arises?
Yep, you should always have a car running perfect before any mods just in case something comes up from the mod...so you don't drive yourself crazy.
Now your stuck, but don't worry, you will be able to figure it out better than the dealer would anyway. Dealers suck for troubleshooting, as soon as something kicsk their butt, they say shit like 'could not reproduce symptom' or 'customer mods causing issue' (which voids your warranty if anything big happens...because it stays on the car's history). Basically, they suck.
What you should do is monitor voltages whith the scanner on and controlling the fan, use an aftermarket volt meter wired directly to the PCM power wire in it's conenctor.
BTW: I had a really weird problem...I flashed a 2006 TBSS last week, the PCM went dead. Turned out to be nothing wrong with the PCM, the problem was a bad PCM fuse connector in the fuse box. All it took was tightening the fuse blocks fuse receptacle pin with the tip of my test light. If that was so loose it caused my problem, I can't help but think it can also cause a high resistance connection that can play havoc on PCM power/voltage. Just something to think about.
I would start by looking for the fan motor ground. Measure the voltage from it to both the engine and negative battery post. You are looking for a high resistance connection. Do the same for the positive side. Possible, as BBA says, there may be a voltage drop where the fans are pulling their current and it affects the FI/coils but not the PCM. There have been issues with GM ign switches as well. (Causing high resistance and low PCM voltages.)
I had a 2002 truck that they shortened the alternator sense wire on because it would produce 'noise' that caused the trans to screw up shifting. So you could disconnect the alternator for a test and see if that changes anything.
(There is a case to be made for extra ground wires as headlights dim when other loads like ABS or load leveling compressors run.)
You can drop a battery/charging system tester on the car and see if the extra electrical load causes the same issues as the fan load.
This could be from being too lean at idle and the extra electrical engine HP load pushes it over the edge.
Last check the voltages to the MAF and tap on it. The vibration from the fans could be causing a bad MAF to mess up.
Thanks for all the recommendations. Looks like I have some work to do now.
For reference, the AC has no effect on idle quality. Only the radiator fan.
By the way, I did not take it to the dealer before installing my mods because some posted that the 08s idle was not as good as previous vettes. And too I had not sat in traffic where the high fan came on to get this extreme bad idle before installing my headers and intake.
Does it miss only at idle or after you have taken off as well?
No misfires with load. Only at idle or a steady cruise in 5th or 6th gear up to about 1700/1800 rpm. In OL no miss fire cruising or at idle unless the fan is on. Although idle quality may be considered acceptable in OL it is not as good as it should be.
Something about OL drastically improves idle to an acceptable level as long as the fan isn't on. Does OL change the voltage of the injectors?
Not any better after a WOT run. Heat makes no difference. I get misfire with a cold engine when I turn the fan on via VCM controls.
I found three ground wires under the hood. Checked the voltage at all three to be the same 12.79. One went into the fan harness so I would assume that is the correct ground for the fan. I did remove all three ground wires, cleaned and reconnected. Same poor idle.
Did you do this with the fan and engine running? 12.79V is battery voltage. You should have 13.5V or higher indicating the charging system is working.
I was thinking to measure the ground wire itself... So from neg batt ground to fan ground then to the engine. Any reading over .5 volt is trouble. (Wire resistance would show up as a voltage.) Since the readings are the same you have it covered though.So any difference of 0.5V with the way you did it with stuff running...
If you unplug the fan motor does the idle improve? (While the fan is commanded on? Not sure if you can safely unplug it running though depending where the plug is.) This would eliminate any computer issues and keep you to the fan motor and it's wiring.
You are almost down to the wiggle test looking for loose wires and connections.
Engine and fan were not running on my 12.79 volt test. Should I do the test with engine running, fan on?Did you do this with the fan and engine running? 12.79V is battery voltage. You should have 13.5V or higher indicating the charging system is working.
I was thinking to measure the ground wire itself... So from neg batt ground to fan ground then to the engine. Any reading over .5 volt is trouble. (Wire resistance would show up as a voltage.) Since the readings are the same you have it covered though.
If you unplug the fan motor does the idle improve? (While the fan is commanded on? Not sure if you can safely unplug it running though depending where the plug is.) This would eliminate any computer issues and keep you to the fan motor and it's wiring.
You are almost down to the wiggle test looking for loose wires and connections.
I previously unplugged the fan and engine idles good with VCM controls at 90% fan. I replugged the fan in and engine immediately idles very poorly.
Tried the wiggle test on every wire connection I could find and all are tight. I'll recheck tomorrow. Maybe I should jack the car and check underside next.
Yes. That is when the problem exists.
Think of a dim flashlight that you bump the switch and the bulb brightens up. Bad connection in the switch. Same thing on the fan off - no load so the wires are fine. Put a load on it with the fan on and a problem wire will show up because a connection can not handle the current. Bumping it is the same as the flashlight - suddenly it can have a good connection.
Your most important measurement is engine on, fan 100%. Measure from a good engine ground to the negative battery post. Then Measure from the frame/fender to the engine. Last Frame/fender to Negative battery post. This checks the 'engine ground strap'.
As you have had headers put on I would look for the engine ground strap, and other small ground wires that may be loose or burned from being too close to the headers. Then follow the rest of the exhaust path just in case they had to move something else to get the headers on. You have already checked the fan ground. (The engine ground strap could be replaced by a cable that runs to the engine, negative battery post, then the frame/fender like the TBSS does.)
It is also possible the fan motor is pulling too much current. (Amp draw test if it comes down to it.)
In any case we are looking for something affected by the fan being on. Pulling down the 12V supply or dumping too much current to the ground so the ground point is at 1V relative to the battery for example. This takes the computer's measured 12V down to 11V. In worse cases the current will flow backwards from a bad ground to a better ground sometimes dimly lighting up the glow plug and other dash lights in a Diesel. (Sparks while cranking tipped that mess off to me.)
Last
You should post a log of this. Perhaps you should bump up the idle fuel mix or other changes some expert tuners may see... This will help confirm if it is electrical. (Some electrical problems are a pain to run down, at least yours is repeatable.)
Sounds like you may be getting somewhere.
WarWagon,
Sure appreciate all the good info!
I installed the headers myself. No wire is on them. I used ties straps and wire loops to insure all wires were away. I did not check for loose ground wires.
I had this problem right from the dealer. I don't think it's anything I have done to the car. Don't even think it's the fan causing the problem because of the bad idle quality in CL regardless of whether the fan is running or not. In OL improved idle and perhaps acceptable but still not as good as my 07. Also the headers seemed to magnify the problem but again they were not the problem just as the fan. I felt no misfire in 5th or 6th gear before the headers. Not sure what the fan effect was before the headers as I never got into traffic where the fan came on high. I guess I was too anxious to make this car go to take care of the problem before mods.
I'm not sure how to bump up the idle mixture. I've tried higher MAF values in the up to 1950 rpm range but to no avail. Also tried different Base Running Air flow numbers. My tuning knowledge is limited.
I'll run your recommended tests and see if I can run down a bad ground.
You said: Quote: I previously unplugged the fan and engine idles good with VCM controls at 90% fan. I replugged the fan in and engine immediately idles very poorly. Unquote
Try using a set of jumper wires to another battery while doing this, as if you were jumping a dead battery. This effectively should give you a double-current battery in the system. If it still goes to a poor idle, something in the fan is causing a VERY heavy load. If it stays at a good idle, your present battery may be weak or not have good connections.
Just something to try.
14.65/14.64 across the battery. Engine to battery 14.65/14.64 volts. Fan ground to battery 14.52/14.53 volts. Approx. .12 voltage drop to the fan ground. Engine idling, fan high speed. Same voltage differences with fan off.
Might add as the engine warms voltage goes down and differences reduce.
Last edited by 8850; 12-18-2007 at 10:46 AM.
Have you swapped batteries from a differnt car yet?
Weak or high internal resistance batteries cause all kinds of weird issues with computer controleld cars...I've seen it first hand.
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