1999 Suburban 5.7 P0302

[kebaile12]

1999 Suburban 5.7L Vortec

Symptoms:
While driving after engine warm-up the engine misfires on cylinder 2 under slight load (basically whenever maintaining speed, and slight acceleration) between 1500-3000 rpm's. Backfires through exhaust when this is happening. Misfire counts on Cyl. 2 collect rapidly and only a few on Cyl. 1 but no P0301.

History:
Purchased vehicle with a blown engine (intake gasket leaked coolant into engine and was hydro-locked) with the intention of using it to pull a boat or camper but never did buy either one. Replaced the engine with a new Goodwrench engine and rebuilt the tranny figuring I would drive it till the body rusted off of it. I honestly never felt that the truck ran as strong as it should have with a brand new engine but just attributed it to being as big as a cruise ship. A couple of years later started receiving multiple codes (don't remember what they were at this time) found the distributor very corroded inside due to the ear on the distributor being broken off. Replaced the distributor, cap,rotor,plugs and wires and fixed it. About a year later, only in the summer I would start to receive the P0302 sometimes, would not happen in the winter. Did the usual plug swap, wire swap, checked spark to rule out ignition with no change. After a while the catalytic converter failed (big surprise with a missing engine) and replaced it as well as both O2 sensors on the rear bank and one on the front bank. P0302 did not change. Last year the fuel pump failed so I replaced it. This April I took the truck on a road trip to visit relatives, about 80 miles out it started backfiring through the exhaust whenever I got over 70 mph and the CEL would flash. Finished the trip babying it the whole way but now it does this all the time when warm.

Repair attempts:
* Distributor, cap, rotor, wires, plugs. (Due to distributor failure mentioned above.)
* Fuel pump and filter. (Due to pump failure mentioned above.)
* Upgraded fuel injection spider. (Because of GM bulletin regarding misfires and multiple failures found on the internet.)
* Repeated valve and fuel system cleanings. (Grasping at straws.)
* Removed rear bank cylinder head and sent to machine shop, All checked good. (After ruling out ignition and fuel figured it had to mechanical. Compression was good, cylinder leak down test was good. The problem seems to be temperature related so thought maybe the exhaust guide was mis-manufactured too tight causing it to drag when warmed up.)
* Crank sensor with a used one from a u-pull it place, I got too much money wrapped up in this. (Found a reference to this repairing a similar problem on another vehicle.)

One final note:
After replacing the fuel injection spider the truck ran extremely well, this only lasted a few days and the problem came back. After replacing the crank sensor the truck again ran extremely well, but only for 1-2 days.
What is the connection between the injection spider and the cranks sensor?

Sorry about the length of the post, just want to give as many details as possible. The only other thing that I can think of that would create a misfire on one cylinder only is the PCM but that gets misdiagnosed so much that I don't want to jump to it unless all other options have been checked out.

Thanks in advance for any help and advice. If something isn't clear just let me know.

[j cAT]

with over 175K miles on this the distributor and the ICM should be replaced. also arcing will cause an old ICM to get damaged . the ICM has a conductive compound that over time looses its ability to conduct heat from the device and then when heated fails. these can be tested at some autozone stores.

[kebaile12]

Originally Posted by j cAT

with over 175K miles on this the distributor and the ICM should be replaced. also arcing will cause an old ICM to get damaged . the ICM has a conductive compound that over time looses its ability to conduct heat from the device and then when heated fails. these can be tested at some autozone stores.

First of all, thanks for the quick response. The distributor was replaced a few years ago (ear cracked on the distributor and corroded inside the cap). I had been thinking about the ICM (although I can't imagine it causing a miss on one cylinder only) and after I posted went to the local u-pull it and grabbed one, not necessarily to fix it but just to see if it makes a difference. No change whatsoever. I'm still open to any ideas!

[j cAT]

Originally Posted by kebaile12

First of all, thanks for the quick response. The distributor was replaced a few years ago (ear cracked on the distributor and corroded inside the cap). I had been thinking about the ICM (although I can't imagine it causing a miss on one cylinder only) and after I posted went to the local u-pull it and grabbed one, not necessarily to fix it but just to see if it makes a difference. No change whatsoever. I'm still open to any ideas!

I do use scrap yard parts but when you do electrical parts it is not good.. is the distributor good ? remove check the rotor/distributor for bearing damage/wear.

[kebaile12]

Originally Posted by j cAT

I do use scrap yard parts but when you do electrical parts it is not good.. is the distributor good ? remove check the rotor/distributor for bearing damage/wear.

Yeah normally I wouldn't dream of using a module from the boneyard, but since I doubted that it was the problem anyway, and I don't necessarily trust the less than experienced parts guys around here to check the ICM correctly, I figured $10 to see if the symptoms changed was a safer bet than $100 for a part I didn't need.
As far as the distributor goes, it was one of the first things I checked, and I've had it out and checked it both when I replaced the spider and when I pulled the head. Gear looks good and bearings seem tight.

[j cAT]

Originally Posted by kebaile12

Yeah normally I wouldn't dream of using a module from the boneyard, but since I doubted that it was the problem anyway, and I don't necessarily trust the less than experienced parts guys around here to check the ICM correctly, I figured $10 to see if the symptoms changed was a safer bet than $100 for a part I didn't need.
As far as the distributor goes, it was one of the first things I checked, and I've had it out and checked it both when I replaced the spider and when I pulled the head. Gear looks good and bearings seem tight.

since you put the scrap yard ICM in see if autozone has the wells electronic module tester working and then test the one you had in there.

so distributor is scrap yard ? http://shop.advanceautoparts.com/c2/...p2HomeClicked=

the miss fires after heating up points to the electrical.

[kebaile12]

Originally Posted by j cAT

since you put the scrap yard ICM in see if autozone has the wells electronic module tester working and then test the one you had in there.

so distributor is scrap yard ? http://shop.advanceautoparts.com/c2/...p2HomeClicked=

the miss fires after heating up points to the electrical.

No the distributor was new. The only scrap yard parts were the crank sensor and the ICM. I only went that way with it because I had serious doubts about either one fixing the problem.
But the peculiar thing is that after replacing the injection spider, the truck ran GREAT, for a few days. Then after replacing the crank sensor the truck ran GREAT, for a few days. I'm not seeing the connection between the 2 parts and a misfire on only one cylinder. I'm hoping someone can enlighten me.

[j cAT]

Originally Posted by kebaile12

No the distributor was new. The only scrap yard parts were the crank sensor and the ICM. I only went that way with it because I had serious doubts about either one fixing the problem.
But the peculiar thing is that after replacing the injection spider, the truck ran GREAT, for a few days. Then after replacing the crank sensor the truck ran GREAT, for a few days. I'm not seeing the connection between the 2 parts and a misfire on only one cylinder. I'm hoping someone can enlighten me.

since you checked the compression on #2 and the other cylinders , replaced plug wires. plugs, fuel injectors then it ran good for a few days still point to an electrical problem. check the wiring and connectors to these timing parts and make sure the ICM is properly mounted with the heat sink compound . the crank sensor also is a part that can get messed up by not seating properly or the connector . I have had these crank sensors fail when they get hot but when they do the engine stops. it cannot be mechanical if it runs for days with no miss fires.

back firing is a timing problem.

[chuck's]

For years I would get a PO305 and random misfire. I found it mostly when climbing a hill and giving just a little more throttle. I did the basics but did not go as far as changing injector. Turns out that that cam sensor timing was out of specs. Adjusted the cam timing and the vehicle worked fine. I hope this will be useful info for you.

[duke350]

I have seen something similar a few years ago. Sold a vortec truck to a friend ( who neglected it ) and about a year after he had it he calls and says its not running right, no power. He can't limp it over so I go to him, its not getting fuel. Hook up fuel pressure gauge and it's well within spec. Start it, pressure is stable, sounds like it runs out of gas. When I owned it I swapped out the spider and still had the orig here. We get it to my place, swap the spider, runs fine. I tell him to make sure he changes the fuel filter. a few weeks go by, same thing. I take apart the spider, see its all mucked up. He stops by, ask him if he changed the filter yet, he says no. I crawl under the truck, pull out the filter, the element has come loose, it was not filtering anything. It was an OEM Delco filter too.

So, basically, I'm saying that it wouldn't hurt to check the fuel filter and injector. Especially if swapping the injector seemed to have made a difference he first time.

Something else to think about, I have read that sometimes out of spec, or even near out of spec valve springs can cause similar issues.