96 Lumina with misfire problems

Hi guys,

I've got a 1996 Lumina that no one can fix! The car misfires often, resulting in a rough idle, hesitation, and a flashing Check Engine light (which, alas, still has not burned out despite being on more than the turn signals)

The dealer has tried to fix this several times - they have now replaced the fuel pump, fuel filter, all the fuel injectors, spark plugs, plus about 2 grand in other misc parts. Still, it misfires. The dealer is blaming it on "the winter blend gas in this area" which apparently only affects Chevy 3.1 liter engines, or only my engine, something like that.

The car does the following, none of which occur all the time:
Misfires, rough idle on start up
Hesitation when accelerating
The above occur more often in cold weather or if the car sits for more than 24 hours without being driven

The following improves the situation, but does not solve the problem:
The closer to full the gas tank is, the less likely it is to misfire
Fuel injector cleaner will typically improve performance, but only for 2-3 tanks of gas
Getting on the interstate and putting the peddle on the floor for at least 5 seconds will resolve the misfires, at least until you have to start the thing again
Misfires typically stop after engine is warm or I accelerate as above
Simply revving the engine with the car in neutral does not stop the misfires

The only other known problem with the car is a faulty thermometer that causes the engine to run 15 degrees COOLER than optimum

I don't want to invest money in this car, but I do need to keep it running for another couple of months or make the decision to junk it and buy a new one. As I'm a poor student, it would be better to keep it running for a few more months.

Does anyone have any ideas as to what might actually be wrong with it?

I'd appreciate any input or suggestions.

Karen

I'd change the Thermostat before I'd do any more guessing. Feedback controlled motors don't like to run cold. What codes is it giving from when the Check Engine light is on?

Is this a Chevy dealer, or just some car lot? Doesn't sound right.

intake leaks and bad fuel pressure regulators are common mis-fire culprits on these engines. If these aren't included in your $2k of miscellaneous parts, they would be my next guess. Try changing the T-stat first like Jeff suggests, as these engines don't like to run too cold. If that doesn't do it, try checking fuel pressure and look for intake leaks.

The engineer in me says the thermostat is unlikely to be the problem since the problem occurs on startup - the thermostat would not seem to come into play. If it were occuring after the engine had been running for a few minutes, I could see that, but looking at the *very dated* basics of engine design I can't see where the thermostat comes into play when the engine is cold - unless the fuel mixture is adjusted for ambient temperature and is thereby too rich or too poor as a result. But not being up on current engine technology, I could be wrong about that.

As far as I can find out (keep in mind my dealer won't let me on the floor to see the actual computer screen) the only information is a misfire of cylinder whatever (it's usually just one, it varies as to which one it is). I'm getting this from Autozone, since they have a handheld that tells them the codes, but I don't know if it gives ALL the information available.

This is a Chevy dealer attempting to fix the problem - not some corner auto shop. The last time it was repaired, I took it from the dealer, drove 2 miles to the gym, came back out an hour and a half later and the thing started misfiring again. I took it back to the dealer the next day and they said it was wax build up from the gas sold in my area, and that I needed their $90 fuel injection cleaning. Since every single one of the injectors was less than 6 months old (including 3 brand new ones), and it had the injectors cleaned less than 4 months ago, I wasn't inclined to buy the "waxy injector" story.

Is there a way to check fuel pressure without taking it to a shop? How would one check for an intake leak?

Thanks

Karen

You are probably right about the thermostat, and I thought the same thing, but after years of working on cars, one thing I've learned is not to ignore an obvious problem thinking it doesn't have anything to do with what you are trying to fix.

The other thing is to never assume that work just performed couldn't be the problem.

I can't tell you how many times a car owner has come in and told me "It can't be ______, because I just had it replaced."

9 outta ten times, they just told me what the problem is. That's always the first thing I verify.

But anyway, your instincts seem to be dead on.

Tell me, is this just a broken, random misfire, like what a lean mixture causes, or a steady rhythmic miss like a dead hole?

The symptoms are typical of a few different things that could be happening, but then other symptoms would say otherwise. Coupled with what has already been done, it's very confusing.

Anyway, about that misfire. What does it feel like?

The misfires are very steady, not at all random. When the car is doing it and it's sitting at a stop light or something, it's a very regular rough idle - like it's only in one cylinder.

Once I get it to stop (the engine gets warm or I can "blow it out" by a very fast acceleration), the engine runs fine.

The reason I decided against fixing the thermostat was the age of the vehicle and that I could figure only one thing that it could really impact short term - gas mileage. If I were to fix all the things that were wrong with the car, I could just take the money and buy a nice used car. Being just out of school with no job yet, I'd rather just have the thing run until I can get a job, and then get another car.

Thanks for your help.

Karen

Went to Autozone today and found the only code is misfires on #6. The problem is now continuous, and I can't get it to stop. Thankfully, #6 is still in warranty from the last repair, so my intention is to take it back to the dealer and get it repaired - again.

Since my hope is to get a job soon and get another car, I'm hoping this holds the car together for the remainder of it's useful life. If not, the thing is up for parts, because I'm not comfortable driving it in this condition.

I know that the dealer will probably just replace the injector again. As long as this fixes it for another couple of months, I'm okay with that.

Any other ideas are welcome in case the dealer can't find the engine or figure out how to get the hood up (not the most competent people on the planet)

Thanks

Karen

well, if you narrowed it down to the 6th cylinder, I would do an injector balence test on it to rule out a bad injector. If the injector is bad, replace it, if not, look into your spark plugs and wires, and possable ignition module and coil packs. Isnt the cylinder 6 and 3 on the same coil pack? I havn't really noticed, but whatever one is, then wouldnt that one be misfiring also if it was a faulty coil pack? I myself would look into just a new set of plugs and wires. Maybe that one wire got hot and melted or something. I wouldnt junk the car over something like that. Wanna sell it? ha ha, just kidding. good luck

I know you said misfire on #6 only, but here's something else to consider: (from your posts, you are very intuitive by the way), you mentioned fuel mixture being adjusted for ambient temperature. It actually is, based on how long your car has been running (open/closed loop) and coolant temperature.

Sometimes bad temperature sensors, don't let the ecm realize that the car is cold, and to adjust fuel mixture for startup, thus running as though the car is in normal operating temp when it isn't. I've seen that cause a bad idle condition. Just a thought...

sounds like a loose sparkplug in #6 hole, seals up when warms up,??????

no,I dont think that a loose plug will act that way. I myself would fix the coolant temp sensor, and then start looking into a new set of plugs and wires.

bad # 6 plug wire would be my first guess. Try swapping 2 plug wires( at coil pack and plug) to see if the miss moves with the suspect wire. If it does, you found your problem, if not, then it's probably a bad injector, or a weak cylinder with low compression. Have they checked compression on #6?

CHECK the 2-wire sensor lead going from the ignition module (just under your 3 coil packs) to the 7x crankshaft position sensor which is on the firewall side of the engine. The wire is impossible to check without pulling the entire wire out of the car. The wires can melt and short to each other or to the engine block. The two wires [yellow and purple] are twisted and 'protected' by 3/8" plastic wire wrap that you commonly find on wiring harnesses.

I'm going to make a separate thread on this subject when I get home from work to help out others with this hard to spot problem.

My symptoms:
I was getting misfires (codes 302-305)
Usually happened in the morning but not always. (probably from dew)
Usually cranked right back up after the engine stalled. I believe the flexing of the engine caused the wires to un-short.
Stalled, lost power, engine felt like it was stumbling along as various cylinders lost spark.
Occurs during OPEN and CLOSED loop!
May work fine for a month or so in between stalls.
After my car permanently died Friday, I got absolutely NO SPARK. You may even suspect it's the ignition module. This lead is the weaker link than the ignition module, in my opinion.

Don't confuse this with the speed sensor which is on your transaxle...they are within 18" of each other and look alike.

You also mentioned that when you press the gas (and increase your RPMs ) that this helps.

this further makes me believe it's the 7x sensor wire because: flexing the engine can unshort the wires and that the 24X sensor (behind the crankshaft damper) helps with spark control over 400RPM.

The dealer may not have been able to detect this unless the engine has failed at the dealer and they detect the no spark condition. NO ONE will actually WANT to check this wire as it is impossible to inspect wihtout taking the wire out of the car!

~kev

The dealer replaced the #6 injector, telling me that it was stuck closed. The only other misfires they found were in #1, but that was only a few and the injector was working when it was there. I examined all the wires and connections before taking it to the dealer and found nothing wrong with the ones from the coil pack on, didn't even know about those back near the fire wall. This was in self defense, as I trust this dealer about as far as I can throw him (which ain't far). Since the first suggestion they made was that it was a wire or spark plug, we had to make a little trip out to examine the car in my presense so they knew I knew that the wires looked fine and so that I could tell them the had to return any electrical parts with "shorts"

Due to the way the injectors keep failing repeatedly, I'm wondering about something in the fuel tank or lines that may be causing this. Even poorly designed fuel injectors shouldn't fail at this rate. Can an injector fail due to an electrical surge? Doesn't seem logical but I haven't looked at the schematics.

I think there probably is some sort of problem in the wiring because of the constant hesitation problems that resolve only when I kick it up to about 400 RPMs, so what you said makes a great deal of sense to me. I don't know that I have the tools to deal with the wire problem, I'll have to look at some kind of repair manual.

I'm still unsure about the temperature sensor, but am extremely reluctant to spend any more money on the car - especially in labor costs for those things I can't fix on my own. So I may look to see if I can replace the sensor.

I seem to recall I looked at an online reapir manual a while back, I'll see if I can locate that.

Thanks for your help.

Karen

I did not need any special tools to replace this crank sensor wire (it could also be the crank sensor itself, if the crank has struck it due to bearing wear) but i would check the wire first...

You will see on the driver side of the ignition module you will have two sets of connectors, it's the smaller, two wire lead (driver side of the car) that goes behind the engine to the crank sensor. You will need a pointed object or screwdriver to pull out the holder that holds the wire in place (you will see it's below the throttle body, about 12" from the end of the wire). I would put the car on jackstands so you can get below the car and also to pull the passenger side wheel for easy access. You can feel for the other end of the wire and gently unsnap the wire from the crank sensor. There is another plastic holder about 12" from the bottom end of the wire - you can reach it with a long screwdriver by going between the transmission oil pan and the bottom of the block... you have a 1" gap there. That holder will probably be very brittle and the tip will break off inside ... you probably can't help that and I dont see any way you can get that little plastic piece out so you can insert the new wire wrap holder (Re read this once you have looked at it and you will know exactly what I am talking about.) Once you have disconnected both sensor ends you can then pull out the wire for inspection.

The only reason I pulled my CKP wire is because it was a little bit melted and brittle near the ignition module... however the most damage was hidden until I pulled the whole wire out.

You can also use a multimeter before removing it to check for a short... but that may not always give you a straight answer.

I havent been able to post pics because my pc at home is down for an OS reload... this weekend I hope to have pics of what to check.

An online repair manual you can use is alldatadiy.com and it's $24.95 for a year subscription. I subscribed after the fact... want to read up on all the TSBs and such....

normally a wiring problem will not resolve itself with RPM. Has any one ever tried doing a compression test on the cylinder with a miss? A leaky cylinder head gasket will cause the same issues with a miss that you are describing, and will resolve itself as RPM increases due to less time for compression to leak out due to faster piston movement as RPM's increase.

normally a wiring problem will not resolve itself with RPM.


In this case it does. The 7x Crankshaft position sensor is mainly needed for low speed revolutions up to 400RPM, and then after that the 24X i believe is mainly used to control spark. A faulty 24X sensor, on the other hand, will not be evident until you go over 400RPM....

I will keep that info in the old noggin, as I have yet to run into that one.

Sir:
I have a 1993 Lumina and had the same problem. If I were you I would go buy a $3.00 spark plug and plug your # 6 hole with your old plug. Than attach the spark plug you bought to the wire for the # 6 cly. and ground that plug with a jumper wire. Now start your car, and see if you have spark to that plug. If not it is either your coil pack, or ignition control module.

In all of you notes you haven't said anything about your coil pack on the #6 cly. Also if it isn't the coil pack get the ignition control module checked. You problem sounds like ignition to me.

Harry

problem fixed yet? Been wondering on this one...

Karen, is the problem fixed? I had similar odd trouble with a 96 Olds 3.1 and may have an easy fix. Let me know. Joe

I don't have jackstands nor do I think I can get the wheel off without an airgun (had to call someone for my last flat - I'm assuming I'd have the same problem with that wheel)

I'm looking for someone around here that has some equipment and expertise that I could barter for - will either find someone or it won't get fixed most likely. Right now it's running okay, and I'm getting used to the hesitation. But your description is so close to what I'm experiencing that I think you're right that there is most likely a problem there.

The only thing I'm not clear on is why the injectors keep failing. All but one have been replaced by a dealer that gives a lifetime warranty on replacement injectors, so he's getting no money when I have to take it in and I doubt he would put crappy injectors in there. The fuel filter has been replaced twice in the last 20,000 miles.

I will probably have to replace the car before winter. If anyone is in the Chicago area and wants it for parts, let me know, because I know I won't get anything in terms of trade in value.

Karen

you can alternately check that sensor wire by just taking off your coil packs, ignition module and reaching in from above with a 1/4" rachet and 10mm socket. You will have to feel around for it. I had already put my car on jackstands and pulled the inner fender panel off because initially I was going to inspect the 24x at the crankshaft damper... but my problem was the 7x on the firewall side of the engine... if you have a small arm, you can lie on top of the engine, left hand can reach teh sensor by reaching below the alternator... or your right arm reaching behind where the ignition module bolts on.

use a thumb to unsnap the sensor lead and it will pull right off. check the wire first, and replace it if it has melted... if it is the wire, you wont even have to unbolt the 7x sensor because you will have found your problem.

worth a check! And I am so glad I finally resolved a problem that has plagued me for over 6 months...