Random misfire

Tiny
5150DEBIL
  • MEMBER
  • 1997 CHEVROLET 3500
  • 7.4L
  • V8
  • 2WD
  • AUTOMATIC
  • 196,000 MILES
Has check engine light on with P0300 random multiple misfire code no cylinder specific codes have checked cap rotor wires plugs changed coil holds steady 20 lbs vacuum, compression is in acceptable range. Had it in for diagnostics they could not figure it out. Only notice at idle runs fine otherwise. New fuel pump. When it goes into closed loop the LTFT goes negative on both banks but more on bank two. Upstream O2 sensors switching as they should downstream reads 8 8 1/2 indicating rich condition as the LTFT does. Fuel pressure at 58 lbs. One thing I still question when checking whats happening with scan tool computer has timing at 24 - 25 degrees at idle this seems to advanced to me. Any thoughts?
Tuesday, October 16th, 2018 AT 12:38 PM

4 Replies

Tiny
MOTOR MASTER
  • MECHANIC
  • 279 POSTS
Hello my name is Dave.

First thing I would check would be the coolant temperature with a scan tool, I have encountered quite a few GM products of that era with those symptoms and ended up being a faulty coolant sensor yet it pulled no related codes, just ran rich. If the coolant temperature is a lot lower than the actual temperature it will cause this problem.
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Thursday, October 18th, 2018 AT 9:58 AM
Tiny
5150DEBIL
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  • 3 POSTS
Will check it out scan tool shows temperature at close to 200.
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Friday, October 19th, 2018 AT 1:51 AM
Tiny
5150DEBIL
  • MEMBER
  • 3 POSTS
Hi Dave,

Scan reads between 197-204 reflected in the dash gauge. Couple things that stick in the back of my mind is the oil pressure gauge ticks like 5 psi little jump for no reason. I had read somewhere that the fuel is routed back through the pressure gauge so if you lose oil pressure it will shut off the fuel pump. Cannot verify that as accurate. Timing at idle is 24 - 25 seems to high to me. Older small blocks I have worked on when the vacuum is reattached your setting at about 18. If I get on it drops to 9 then up to 32 settles down at 27.5.
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Friday, October 19th, 2018 AT 10:06 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,144 POSTS
Hello,

I have seen the MAF sensor cause this problem. I would replace it because by this time cleaning it will not help. You can get a new one from Amazon. Here is a guide to help you replace it. The sensor will be off a little but not enough to set a code.

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Please run down this guide and report back

Cheers, Ken

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Monday, October 22nd, 2018 AT 1:07 PM

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