Odd Stumble After Intake Gasket Change

shorgasm · December 17, 2007

1999 Denali 5.7.120k on her.

Friday night she blew the intake gasket out pretty good and heated up to about 230ish but made it home.Never got past the 3/4 mark and only briefly flashed the check gauges light.It had a slight leak for over a year but it was so small it never even required coolant till you used the heat.You would never see the coolant just smell it.I knewit was coming though.

Anyways, got the intake on and all seems well as we have no leaks and no heating up past 190 or so.That part went well.

We did have a great deal of trouble getting the dist. right.Got her running pretty good last night but just a touch rough idle.Nothing new though.Occasional idle stumble but got gradually better.We had run the battery down with the lights on.The bad part was at partial throttle acceleration it would stumble.Full throttle and it ran out fine.Still needed plugs and wires.I put a new cap rotor on last night.SES light was on from the second we started her up.Went to AZ and they pulled a MASS code and P1345 misfire.I knew we had the dist wrong or at least close.Got on here and read about the 8 mark and we re-did it.Fired right up and great idle.No idle stumble at any time.SES light still on.I dumped the water in the rad and put fresh coolant, as I went back to check the temp the SES had gone out.Never came on again.Changed the oil/filter and put new plat plugs and wires on.No issues what so ever.Now everything is new..cap/rotor/plugs/wires.All routed correctly and put back in there respective holders.

Cranked right up and smooth idle with no issue at anytime.Smoothest it has idled since I have owned it.Took it for a drive and I'll be damned, the stumble is still there.BUT, it more intermittent now and less noticeble.Only light throttle acceleration will produce it and about half the time it won't even do that.It feels like it cuts out ever so lighlty.No backfire or popping at all.Drove it for about 20 minutes and the stumble got less and less but at times it was still there.Even when it would stumble,I could push the pedal down farther and it would go like normal.WFO never a issue.Idle never an issue.No SES at anytime.

I'm stumped guys..any ideas?

klunkerbus · December 17, 2007

Sounds similar to something I just went through on our 03 Malibu. After a lot of maintenance work including new OEM plugs and NAPA wires, the car would occasionally stumble at idle or initially on acceleration. No check engine light ever came on. Turns out that my problem was the NAPA Gold (by Belden) wires. After putting on AC Delco wires, the car is fine. Just a thought...

shorgasm · December 17, 2007

Sounds similar to something I just went through on our 03 Malibu. After a lot of maintenance work including new OEM plugs and NAPA wires, the car would occasionally stumble at idle or initially on acceleration. No check engine light ever came on. Turns out that my problem was the NAPA Gold (by Belden) wires. After putting on AC Delco wires, the car is fine. Just a thought...

Yes, that sounds reasonable except for the fact it is the same stumble that was prior to the wire and plug change.The only thing that improved was the quality of the idle, as in smoothness.BTW, it had the original plugs/wires/cap/rotor on it.Hard to ignore the old part number on the plugs from GM way back when.

I almost want to say the truck is re-learning itself.The stumble is less and less each time it seems.

shorgasm · December 18, 2007

Any other ideas guys?

Maybe EGR got dirty when cleaning the manifold?

I've never had any issue out of this truck other than a fuel pump.*knockonwood*

The intake runners on the manifold where nasty looking and caked up with black but the injectors where spotless shiny at or around the tips.

No build-up of anything in the valley though...no sludge no debris and the oil change showed the oil to be dry and relatively clean.

ChrisNJ · December 18, 2007

Back in the day I did a lot of my own and for freinds auto repair. My experience with any aftermarket parts from many different sources was poor. I went to oem parts, paying more, but the success rate went up exponentially. Since your getting better results over time I would just let it play out a little and maybe the computer will fix it.

turfy · December 22, 2007

Spray some carb cleaner around the throttle body and see if the idle smooths out. If it does it is a vacuum issue and the car is to lean. Check the gasket or O-Ring in the area that makes it smooth out

jcsinc · December 29, 2007

shorgasm,

Did you figure out your stumble problem? Yesterday I replaced my injector, upper intake gasket and plugs on my 97 yukon @ 155k. It also has a stumble or rough idle at stops but it runs great at normal driving speeds. Today it threw code P0171, let me know what you come up with or I will let you know if I beat you tooo it.

jcs

jcsinc · December 31, 2007

Shorgasm,

I found my problem, thanks for the post on how to find a vaccum leak with carb cleaner.

I had a vaccum leak at the rear passenger side of the upper intake plennum (the one I removed to install the injector assembly) and I forgot to tighten down the studs at the corner and under the coil pack.

If I had gone to the stealership or mecahnic to have the injectors changed, they may have charged me extra for the vaccum leak and try to convince me, the intake work had nothing to do with the leak. Yall know how they treat the avg. non-mechanical customer.

Thanks again

jcs