1999 SL2 Misfire Mystery

[SL22]

I have a 1999 SL2 with a misfire at idle. The previous owner changed a bunch of parts. He told me what he changed and he left all the old parts in the trunk. I changed some parts and ran some tests and still have the misfire at idle.

No misfire on the highway, I cleared the code and drove about 50 miles on the highway, refreshing the reader and got no code. As soon as I pulled off the highway and stopped at a light the misfire and the p0301 code came back.

Occasionally, the engine will run good with no mis at idle, but recently the misfire is more consistent, happening every time the car is idling.

The previous owner did the following,

- replaced egr valve
- I believe he told me he replaced the ICM but I will have to check the trunk to be sure. I know there is an engine computer in there
- replaced the first cyl injector
- replaced coil pack
- replaced plugs and wires (which I did again)

what I have done is

- replaced spark plugs to recommended NGK plugs and NGK wires. Checked for spark and got good spark across all 4.
- ran a compression test which came back good, about 190 accross all cyls
- checked for vacuum leaks which I did not find. My code reader is getting readings around 19 and 20 in/hg for vaccum
- unplugged the egr valve which did not change the idle
- ran a bottle of Techron in a tank of gas to clean the injectors
- cleaned the throttle body
- changed motor mount to recommended wester mount



I have also purchased a new filter/regulator to put in. I am waiting on it to come. I am also planning on taking the O2 sensor off and seeing if the open exhaust helps at all.

So far have not been able to get any change in the misfire. Wondering if anyone has any advice for.

Also wondering if I cannot find the misfire, how long can I reasonably expect to drive it before it causes more damage? I am happy with the performance and mpg of the car and going to keep driving it as my commute vehicle even if I cant fix it. Would love to get it fixed though.

Thanks in advance for any help.

[SL22]

Forgot to mention I also changed the ECTS. The one in there was the brass one but I put a new brass one in there anyway.

[billr]

Is the code always P0301, no others ones? If so, try swapping the #1 injector with another one.

Much of what you mentioned (ICM, coil, fuel filter, EGR, O2 sensor, ECT. TB) will not affect only one cylinder. With #1 having new plug and wire, and compression being OK, I suspect this is a bad injector. If you do swap injectors, peek at the P/N on each. Make sure they are the same. The P/N will be molded into the plastic connector housing, very difficult to see with injector on the engine.

[SL22]

Originally Posted by billr

Is the code always P0301, no others ones? If so, try swapping the #1 injector with another one.

Much of what you mentioned (ICM, coil, fuel filter, EGR, O2 sensor, ECT. TB) will not affect only one cylinder. With #1 having new plug and wire, and compression being OK, I suspect this is a bad injector. If you do swap injectors, peek at the P/N on each. Make sure they are the same. The P/N will be molded into the plastic connector housing, very difficult to see with injector on the engine.

Will do. I did check and make sure it's a new injector in there. The previous owner forgot to put the clip that holds to the injector to the fuel rail back on so I did that but no change. Was going to swap the injectors on Saturday but didn't have the time. Will do that this week.

As far as the fuel filter/regulator, I figured maybe since it's the last cylinder on the rail it's not getting enough pressure at idle? The car has 90k so it's due to be changed soon anyway and it was only $27 shipped on Amazon.

Thanks for your help!

[SL22]

Also to answer your question, test it's only p0301. I sometimes get p0300 with it but I have never gotten a code for another cylinder.

[billr]

It may be a new injector, but still may not be the correct P/N; be sure to check the P/N anyway. And, of course, "new" does not ensure that it is "good"...

The passage in the rail is huge compared to flow through all injectors combined. I doubt there is any (reasonably) measurable difference in pressure from end-to-end.

Yeah, the P0300 may pop up if any cylinder is misfiring. No surprise there.

[SL22]

Originally Posted by billr

It may be a new injector, but still may not be the correct P/N; be sure to check the P/N anyway. And, of course, "new" does not ensure that it is "good"...

The passage in the rail is huge compared to flow through all injectors combined. I doubt there is any (reasonably) measurable difference in pressure from end-to-end.

Yeah, the P0300 may pop up if any cylinder is misfiring. No surprise there.

Would a bad injector be likely to cause a misfire at idle that goes away when driving?

[billr]

It certainly could. Two different scenarios:

1) Injector is leaking slightly. The extra fuel just makes the mixture a bit rich when engine speed/load is up, but makes idle too rich to fire.

2) Injector is sticking, so opening is delayed, again slightly. Mixture at idle is affected most (going lean) because the idle injector pulse-width is almost all opening/closing time. Yeah, a sticking injector might stay open a bit longer, thus canceling out the long opening time, but usually they close easier than they open. The high-Z vs low-Z debate relates to that in some ways.

[SL22]

Thanks for all the help so far.

I used a stethoscope to check the fuel injectors for clicks and the cyl one sounded good. No discernable difference from the others. Still thinking I will take it off and swap it with another cyl when I get a chance.

I had a bottle of RESTORE additive that I was bought when I picked up the compression test gauge. Didn't end up using it because the compression was good. Was going to return it but I was about a half qt low on oil so I decided to throw it in and see if it would have any effect. Also have a bottle of MMO and planning on doing a piston soak but don't know if that has ever helped anyone with a misfire issue.

I drove for about 35 miles to let the oil mix up and give the additive a chance to do something. The whole time with the AC on. Stopped to get pizza, checked the codes, no codes. Went back to the tail pipe and it sounded like it was running good with no misfire.

Drove the car home and the light never came on. Got out and checked stood by the tail pipe, no misfire sounds. Was going to leave it running for a while to see if the light would come on so I turned off the AC. Immediately could hear the misfire from the tailpipe. Turned it back on and let the car idle for 30 mins. Light never came on. When I have let the car idle in the AC off the light comes on fairly quickly.

Wondering why the AC makes a difference. I think the most likely scenario is that the slight increase in load and idle RPM is negating whatever is causing the misfire.

Anyone ever run into something like this?

Wondering If I can somehow increase the idle RPMs to stop it from misfiring. Currently the car idles at 850, so I have some room. I read that the idle adjustment screw on these cars should never be touched. I am considering purposely causing a pin hole vacuum leak on a replicable hose to get the idle up to about 950 and see if the misfire persists or not.

Also wondering what the general consensus is on how long the car will run like this. If I can't fix the misfire soon and cheaply I am just going to leave it and drive it the way it is. I got a great deal on the car so if I get even a year out of it ill be happy. Wondering if this will kill the car sooner or later before I decide if I am going to get new or used tires lol.

Thanks for all the help!!

[billr]

If the IAC system is working correctly, you won't accomplish anything by adding in a vacuum leak or opening the TB stop-point until the IAC is fully closed, has run out of authority. Opening the TB would also upset the TPS reading, which affects several aspects of the engine/car operation. I would not do either of those

Listening for injector "click" is a pretty crude test, will catch only a totally inop injector. Probably won't catch the leak/stick I mentioned earlier.

A/C requires enough engine power that the fueling is probably out of true "idle", the effects of injector problems at low flow are reduced. No big surprise there, either. You may be able to produce the same effect by turning on all possible electric loads (heater blower, all lights, engine fan, etc) and loading the alternator. I think forcing the engine fan on can be done by disconnecting the ECT sensor cable.

[SL22]

Thanks! Looking to switch the injectors some time this week and see what happens.

I forgot to mention that yesterday when driving I saw a pending code for low voltage to an O2 sensor. I didn't save the code number or the info it gave and I wrote it off thinking that and O2 sensor probably had nothing to do with the misfire and I had not seen it come up before. I cleared it and it didnt come back.

I was searching around the forum and saw this thread https://www.saturnfans.com/forums/sh...d.php?t=183571

Now I am thinking its possible it might be related, but think its still unlikely because this problem would probably affect all cyls not just the 1st, and I have only seen the P0301 code.

Any opinions are much appreciated.

[billr]

I agree, I would expect an O2 sensor problem would affect all cylinders; but all do not fire exactly the same. Maybe #1 is simply the first one to be too poor to always fire. Keep an eye on the O2 sensor, and resolve that if it persists. Always fix known problems to eliminate them from the circus.

[SL22]

Have you heard of anyone having any luck fixing a misfire with a Marvel Mystery Oil piston soak? I know it's recommended on this and other forums for dirty engines and engines that consume oil but has it ever fixed a misfire?

Haven't had time to swap the injectors and don't think I'll be able to do it tomorrow. Might be able to put some MMO in the plug holes and let it soak until Sunday though.

[billr]

I just posted on another thread, piston soaks are a "Hail Mary" play to reduce oil burning. I don't think I have heard of them curing misfire. I can tell you my two oil-burners have no misfire and the plugs keep clean.

[SL22]

In the middle of swapping injectors and the o ring from number 2 fell into the port on the manifold. How bad is this? Is this an absolute must remove the manifold and take it out? Or is there a chance it might be fine?

Not trying to turn this into a huge job so hoping it's not the end of the world.

Thanks.

[SL22]

Also the part number on the back is 3290 25176913. The part number on the others is different. I'm thinking it might be a SOHC fuel injector? I googled but all I could find is that it is for a Saturn 1.9l and doesn't distinguish between SOHC and DOHC. Are the fuel injectors compatible amongst both engines?

[Waiex191]

DOHC injectors have more flow.

[SL22]

Do you know if that part number is a DOHC or SOHC injector?

Also any input on the O ring that fell in there? Thinking I probably need to take the intake off but hoping the consensus is that it will probably be fine.

[billr]

Sorry, I recommend getting that O-ring out of there. It might just "blow through", but the potential for ugly damage is too great to risk.

I gotta ask... how did the O-ring get into the manifold??? The holes for the injectors should be smaller than the O-ring OD. Or is the fuel-rail ring a smaller size than the one for the body?

I will see what I can find about that injector P/N. What is the P/N on the other three?

[billr]

I find the 25176913 is a high-z injector, 13.8 #/hr @ 3 bar (43.5 psi).


Get me the other number and I will see what I can find for it...