2001 GMC Jimmy multiple misfire code

Tiny
SWEETLEAF
  • MEMBER
  • 2001 GMC JIMMY
  • 4.3L
  • V6
  • 4WD
  • AUTOMATIC
  • 180,000 MILES
Ok here's the back story, my aunt got this truck from a friend who took it to the dealer to have it checked before giving it to her, and the tech told them it was ok but shouldn't take it on long trips. So I take a look at it, the check engine light was on, had code p0300, and ran smooth at idle, but felt misfire when under load. Also noticed the oil pressure gauge read 25 psi at idle when hot and 60 when under load (which, in my experience, is about right with gm gauges) Replaced plugs, wires, cap and rotor, cleared codes. Truck ran great, no misfire. She drove it for a few days and the check engine light came on, I scanned it and it came up with p0300 again, but was running smooth at idle and under load, I didn't have time to look at it further so she drove it from Richmond va to baltimore md and back (about 300 miles) and when she got back it had a really bad rod knock. So I replaced the engine with one from a junkyard {the vehicle was wrecked and was told the engine ran) now the new/used engine runs great but the check engine light still comes on with p0300 code, I have a matco scanner that reads real time data and it shows misfire on cyl # 1 & 4, more times on 1 than 4, I replaced the ignition module in the distributor on a hunch, but didn't change anything, I'm going to check the CKP and do a fuel leak down test, since I heard they could be probable causes. My question is, how can I check for a weak injector or do you think the rod bearing in this engine is starting to go bad as well, if so, how can I diagnose it, without pulling the oil pan
Do you
have the same problem?
Yes
No
Thursday, November 27th, 2014 AT 9:56 PM

3 Replies

Tiny
CARADIODOC
  • EXPERT
GM has a common problem with injectors at high mileage that can cause misfires and unusual fuel trim numbers. Since you were fortunate to get cylinder-specific fault codes, switch some of the injectors to see if the misfires change to different cylinders.

The injector problem relates to flow rates. Chrysler buys their injectors from Bosch in flow-matched sets. GM grabs a handful out of a bin and throws them in an engine with no regard to flow rates. I don't know what other manufacturers do but I was told this is strictly a GM problem. For more information you can do a search for Jim Linder. He has a fuel injector rebuilding company in Indianapolis, IN and he puts on very high-level classes around the country, typical for hundreds of people at a time. We were lucky to get him at my community college a few years ago, and this is where my information comes from. He also lists the top ten injector problems on his web site, and as I recall, eight of them are for GM products.

With wear and some varnish buildup over the years, some injectors won't flow as much fuel as the others. That could cause a lean misfire, then the oxygen sensors will detect the excessive unburned oxygen in the exhaust and the Engine Computer will command more fuel to correct what it thinks is a lean mixture. One clue is if you watch the short-term fuel trim numbers on your scanner, you'll see high positive numbers meaning the computer is adding fuel above the normal fuel metering calculations.
Was this
answer
helpful?
Yes
No
+1
Thursday, November 27th, 2014 AT 10:41 PM
Tiny
SWEETLEAF
  • MEMBER
Hey Caradiodoc, thanks for the reply, especially on turkey day. I appreciate all the advise and info, especially the "GM grabs a handful out of a bin" part. Unfortunately, you can't just swap injectors on this as easily as others. Fortunately, since I have the old one, I can check and or clean them, and swap out the whole unit. Which is what I'm gonna do, after a few other tests I found I can do. I will also be checking the STFT settings also, thanks for that. I checked on Jim Linder, apparently, he's retired now, and his website for to 'bubba' hot rod shop, ' so I'll check that a little more also.
Was this
answer
helpful?
Yes
No
Friday, November 28th, 2014 AT 10:27 PM
Tiny
CARADIODOC
  • EXPERT
Boy, is that disappointing. The street address is the same so the new site must be for one of his employees. Too bad. There was a real lot of good information there.
Was this
answer
helpful?
Yes
No
Saturday, November 29th, 2014 AT 11:04 PM

Please login or register to post a reply.

Recommended Guides