Severe Stumble Intermittingly on my 90

[moesvette]

My 90 is getting a severe stumble which feels as though I am getting no fuel every time it happens. Has never had an issue to this point. New injectors, new plugs about 1 year ago, and has run flawlessly up til now. Everytime the stumble happens I see the Check Engine light come on briefly. It simply feels like a fuel delivery issue, and am wondering if the fuel pump could be the culprit? The first several times it did it, I drove for awhile and it stopped doing it and drove fine from the rest of the drive. The last time out was by far the worst, and pretty on and off stumbled until I got it back home. Let it rest and fired up again, idled it at 2500-3000 and never a stumble. I have 35,000 miles on it, and it has always been garage stored. Would it be a good idea to change out Fuel Pump if for no other reason as preventive? It seems as though I have read about many fuel pump issues. Any help/suggestions would be most appreciated.

[Kevin]

Sounds like the same problem I'm having with mine. Have you checked the codes? Mine was code 61, vac leak. I've narrowed it down to a bad check valve or secondary solenoid under the plenum

[moesvette]

No haven't checked the codes yet. I was hoping it wasn't vacuum related. I will pull codes and see what I get.

[Kevin]

pulling the codes is the best place to start, that way we can focus on what's wrong vs just guessing and hoping for the right answer. If it is vacuum it's not a huge deal. Still easy to sort out. Let's hope it's something simple and we can get you up and running again soon.

Has any work been done to the car recently? Does it run better with the key off vs on? Mine runs fine with the key off but stumbles like crazy with the key on.

[Z06scentair]

Make sure the plug wire boots are filled with Di-electric grease. Sounds like an arcing mis-fire.

[Schrade]

Fuel pressure; static, dynamic, and leakdown time. Dynamic with vac supply to FPR on, and off...

My 90 is getting a severe stumble which feels as though I am getting no fuel every time it happens. Has never had an issue to this point. New injectors, new plugs about 1 year ago, and has run flawlessly up til now. Everytime the stumble happens I see the Check Engine light come on briefly. It simply feels like a fuel delivery issue, and am wondering if the fuel pump could be the culprit? The first several times it did it, I drove for awhile and it stopped doing it and drove fine from the rest of the drive. The last time out was by far the worst, and pretty on and off stumbled until I got it back home. Let it rest and fired up again, idled it at 2500-3000 and never a stumble. I have 35,000 miles on it, and it has always been garage stored. Would it be a good idea to change out Fuel Pump if for no other reason as preventive? It seems as though I have read about many fuel pump issues. Any help/suggestions would be most appreciated.

[Kevin]

Fuel pressure; static, dynamic, and leakdown time. Dynamic with vac supply to FPR on, and off...

Lets wait till he pulls the codes and we see what they are.

[moesvette]

Both good points, and I have not tried running it without secondary key in off position. I actually am a little nervous to take it out for a drive after the amount of stumbling I saw before, but it would be worth trying it without the secondary key on. Curious though, how if I have a possible vacuum issue, why it will run for periods of time perfectly and then only intermittingly stumble?

[Schrade]

I suggested that Kevin, since he was thinking of replacing the fuel pump, without checking pressure...

Yeah - codes FIRST (I missed CEL note); OP; codes are in History DTC Hxx Module (1?) [ECM]

Lets wait till he pulls the codes and we see what they are.

[Kevin]

Both good points, and I have not tried running it without secondary key in off position. I actually am a little nervous to take it out for a drive after the amount of stumbling I saw before, but it would be worth trying it without the secondary key on. Curious though, how if I have a possible vacuum issue, why it will run for periods of time perfectly and then only intermittingly stumble?

no idea but mine does the same thing. runs fine cold, soon as it warms up it stumbles and sets a code.

[moesvette]

Ok, so I tried to pull codes last night (wire to pin K, and grounded to frame). I got nothing....no 12 flashes, nothing. I have never tried to pull codes before, so could be my inexperience, but I tried multiple times, even grounding to negative term of battery. Any suggestions? I did not use a paper clip in the K pin location, simply stuffed the wire in, but was sure I made contact with the brass part of the pin. HOWEVER, interestingly enough I just happened to check the battery cable to positive side of battery and it was loose. How stupid I am if that is all that was wrong. Tightened and fired up for 20 minutes at different idle revs, and never missed a beat. Couldn't take a drive as weather was bad so that will be tonight. Also checked fuel pressure: Static 48 psi, and after starting engine 42, when revving, it jumped up to 50. Thank you Shrade for the suggestion. Any ideas on how to figure out if I can pull codes?

[Schrade]

Sounds good; looks like your FPR is responding properly to vacuum too.

Dash LED still should show testing tho', when you jump the ALDL connectors. I think first 2 pins should initiate the diagnostics

http://www.corvettebuyers.com/c4vettes/84_93_code_display.JPG

AC/Delco batteries had a problem with battery sockets stripping out of the case - maybe put some dielectric grease on cable screws into battery for good measure...

[XfireZ51]

no idea but mine does the same thing. runs fine cold, soon as it warms up it stumbles and sets a code.

Sounds like once it goes Closed Loop, then ...
Any chance you observed coolant temps when that occurs?

[Kevin]

Sounds like once it goes Closed Loop, then ...
Any chance you observed coolant temps when that occurs?

i was looking at the oil temp so no. Let's keep this on the TS problems. don't want to clutter up the thread.

[Kevin]

everyone says use a paper clip but if you're like me you don't even want to eat a sandwich you made yourself because it has blood and bits of finger in it. buy the fault grounding plugs from gordon https://www.gordonkillebrew.net/Products top right. best $45 ever

[Paul Workman]

Sounds like once it goes Closed Loop, then ...
Any chance you observed coolant temps when that occurs?

i was looking at the oil temp so no. Let's keep this on the TS problems. don't want to clutter up the thread.

Say what? A perfectly good question, methinks! Open/Closed loop would certainly be of interest, Kevin, especially since CLOSED loop brings the O2*s into the picture.

*BTDT: A faulty O2 on one of my Vettes resulted in mild to severe stumbling, after going CLOSED loop.

[Kevin]

Say what? A perfectly good question, methinks! Open/Closed loop would certainly be of interest, Kevin, especially since CLOSED loop brings the O2*s into the picture.

*BTDT: A faulty O2 on one of my Vettes resulted in mild to severe stumbling, after going CLOSED loop.

I didn't want to clutter up this thread with problems with my car. I've got a handle on them, more or less.

[mike100]

My car has a ton of new parts in it and I recently had a weird hesitation coming off of coast to light throttle tip-in. The only thing original is the ign wire/coils which I was starting to suspect, but I did examine the plugs, and checked the fuel pressure was 43-52 as it ought to be etc etc. full power was ok, cold running ok...

Long story short it turned out to be a bad tank of gas... (too much ethanol maybe-I'll probably never know).

[moesvette]

So....am I supposed to jumper A & B, then ground K to get the codes? I only tried grounding pin K. The idea of getting the jumper from Gordon is a great idea. Will get it ordered. Thanks to all for the help. I hope the battery correct will solve the issue. I will post after I have had a chance to validate.

[Kevin]

So....am I supposed to jumper A & B, then ground K to get the codes? I only tried grounding pin K. The idea of getting the jumper from Gordon is a great idea. Will get it ordered. Thanks to all for the help. I hope the battery correct will solve the issue. I will post after I have had a chance to validate.

may want to throw his hood opening tool into the order since you're there

[moesvette]

Thanks for all the help.........turns out the issue was certainly the loose Positive battery connection as it has run perfectly ever since. Appreciate the help from the forum.