Picked up my car from the body shop Friday (had some primer bleeding through the paint for some reason) and it was running fine. It's about a 15 minute drive back to my house. I got about halfway home and the car didn't feel just right. I've had a little vibration since I changed the torque arm and I just figured it was getting worse. Got on it a couple of times and it felt stronge. I pulled in my driveway and at an idle the whole car was shakeing. I get out to listen and it had a dead miss on several cylinders.
Today was the first time I've had to work on it so I put the ATAP on it and #5,7 and 8 were missfireing to the tune of about 2700 a piece in about 10 minutes of run time. This was in open loop so I'm ruling out the O2's. Next I checked for vacume leaks. I'm not ruling that out but I didn't find any. At any rate a vacume leak wouldn't cause it to missfire on specific cylinders like that would it?
After that I pulled up the DTC freeze frame data and the only thing that looked strange was that it showed fuel pressure to be 0.0psig. First off I can't find anywhere in atap to display the fuel pressure so how did it record that dat in the Freeze Frame? Second off what is psig?
Anyway, so I started trying to trace down a fuel pressure problem. I installed a fuel pressure gauge the other day but I needed an adapter from the scroeder valve on the fuel rail to a pipe fitting for the sending unit. Called AutoZone to see if they might have one and no one there even knew what a scroeder valve was So I pushed the valve in with a pocket knife and fuel shot everywhere. Now while I know this isn't the best way to check fuel pressure I did find out that there is some fuel getting to the fuel rail, maybe not enough to run properly at WOT but it should at least idle without missing.
Checked the grounds for the PCM and they are hooked up and tight. Unless the wire in the harness is broken the PCM should be grounded.
Swapped sparkplugs on #7 and #1 and the missfire didn't migrate so that rules out bad plugs. Did the same with the plug wires and that didn't work either.
Was going to change coil packs with a cylinder that was not missing but I didn't have a 10mm deepwell.
Tomorrow I'm going to change the coil packs and check it again.
Then, check the wires going to the injectors. GM has a specialty tool to plug in to this and it flashes a light when the injector would fire, is that a 12V circuit? Can I check it with a standard test light?
Last I'm going to swap injectors with a good cylinder and see if the missfire goes with it.
Any other suggestions?

Thanks for the help
Dave

 

ttt for ya!

 

Thanks for the TTT Sparky. Come on guy's help me out.

Little more info. I sprayed the back of the motor and the vacume lines down with either and no change in RPM so I'm ruling out a vacume leak. On the bright side, the motor's clean now <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

When I started the car this morning it ran good for about five to ten seconds and then started missing so I'm thinking it's fuel pressure related. I need to find an adapter for the schroeder valve. Anyone know where I can get one.

Dave

 

I don't think fuel pressure is your problem, if f/p is low hard starting bogging down under accel would be your most likely complaints. DO NOT TEST ANY PCM CIRCUIT WITH A TEST LIGHT!!!! If you want to test for injector pulse, you'll need a injector noid light (there are different ones get correct size). A vacuum leak will has the greatest efect on idling. Does it only miss at idle? I would look for a vacuum leak and a big one at that to kill 3 cylinders. Don't trust every pid atap throws at you as gospel.Start with the basics and go from there. Check for vacuum leak again this time with propane bottle w/torch end off and a 1/4 vacuum hose on it, turn bottle on & sweep all suspected areas , the propane gas is dense so it will sink and lay in area untill heated then it slowly disapates. If hit a vacuum leak you'll know it. this is a much better method of finding leaks.

 

I don't think fuel pressure is your problem, if f/p is low hard starting bogging down under accel would be your most likely complaints. DO NOT TEST ANY PCM CIRCUIT WITH A TEST LIGHT!!!! If you want to test for injector pulse, you'll need a injector noid light (there are different ones get correct size). A vacuum leak will has the greatest efect on idling. Does it only miss at idle? I would look for a vacuum leak and a big one at that to kill 3 cylinders. Don't trust every pid atap throws at you as gospel.Start with the basics and go from there. Check for vacuum leak again this time with propane bottle w/torch end off and a 1/4 vacuum hose on it, turn bottle on & sweep all suspected areas , the propane gas is dense so it will sink and lay in area untill heated then it slowly disapates. If hit a vacuum leak you'll know it. this is a much better method of finding leaks.

 

Wrencher, thanks for the reply. I found a noid light and the injector harness is fine. Miss is not only at idle. I didn't want to get too caried away but it will miss up to 3000 RPM at a cruise. What's strange is one time you can hit the gas and tach it up quick and it will clean out and not miss. The next time you do it it will spit and cough. That seemed kinda weird.
I'm not sure what your talking about the pid "Don't trust every pid atap throws at you as gospel"
I like the idea of useing a propane torch with a hose. The one thing I couldn't do is spray the either up under the back of the intake. Which I would like to because of the cylinders that are missing. I still doubt that it is a vacume leak, I used John Deere starting fluid which is 80% either. That stuff is way better than anything else you'll find. You can just barely spray it in the intake and the RPM's will jump about 500. While I couldn't spray it up under the manifold I sprayed direcly into the fire wall enough that there was a cloude rolling out the other side. Like you said if it's a leak it would be a big one and it should have got some of that.

 

I hate to say it but your miss sounds like ignition to me. If it is abrupt & sporratic as you say it's gotta be ignition. Wet down plugs & wires with a spray bottle & see if it gets worse. A pid is a data perameter from a scan tool, and the only one that is designed for are vehicles is the tech 2 a gm scan tool & big $$. Other "scan" tools are generic by compairison & so I say don't take there pid's as gospel ,have seen aperfectly fine pcm with either slow changing or inaccurate pid's due to the "scan" tool. The realy don't like to spray paint damaging/flamable agents in the pursuit of a vacuum leak +propane works great. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> and no fires! <img border="0" title="" alt="[Razz]" src="gr_tounge.gif" />

 

Thank's for the tip. Tried the propane this morning and still nothing. I think I can safely eliminate a vacume leak. I'll try the water bottle.

 

not tryng to scare the heck outta ya, but when i did current misfire history on mine i had like 80-90 misfires on cyl4 and 6 would also so lots of misfires. Cyl close to ones thats have issues will show misfires. Its the way the PCM trys to find them so thats how they show up on atap.

My misfires were from a broken spring. I had a cracked into 918. Didnt even have 200 miles on the spring. But your car should run like crap in the upper rpms i would think from a broken one

 

Hey, another farmer on here. Cool.

Yea, I knew about the missfire thing. I've got a small amount of missfire on every other cylinder but #1. I haven't been concerned with that because of what you just said. When I get the three bad ones straightened out then I'll address the others.

I hate to say it but I'm starting to think it's internal. Maybe it will be something simple like a broken valve spring. I can deal with that. Probably is D@mn CarTek heads.

<small>[ August 28, 2002, 10:22 PM: Message edited by: DBerry ]</small>