My K2500 Chevy Suburban 7.4L runs fine cold but within 5-10 miles starts to falter.

Tiny
DMCCRACK
  • MEMBER
  • 1998 CHEVROLET SUBURBAN
  • 160,000 MILES
No codes are being set. When it falters the tachometer drops way off in tune with the (misfires?). I can barely keep it at 70MPH. I have checked so many things. Let me just say the fuel pressure is good, the plugs/wires/cap/rotor are new, engine ground and battery cables are new. New sensors: O2 (4), MAF, crank position and camshaft. All Delco parts (and probably more than I am remembering to mention).
Friday, October 5th, 2012 AT 4:40 PM

8 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
What is the fuel pressure. Also try tightening your intake bolts down if they are 10mm heads. They loosen up with that motor and cause a vacuum leak. It shold be 60-66on a CSI system or 56-62on SFI check for a vacuum leak by apraying choke cleaner around manifold. Clean throttle plate with choke cleaner on both sides as well. If you have a J in vin that is engine code for SFI
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Friday, October 5th, 2012 AT 4:55 PM
Tiny
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Thanks for the reply. I'll have to borrow or buy a tester that I can read while driving.
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Sunday, October 7th, 2012 AT 9:02 PM
Tiny
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Everything can be done with just turning thekey on not driving
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Sunday, October 7th, 2012 AT 9:03 PM
Tiny
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While I'm at it, is the ECM module's reported psi good enough to use, or do I need to get a tester? Also, what Is acceptable Vacuum on this engine?
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Tuesday, October 9th, 2012 AT 4:38 PM
Tiny
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20" at idle if at 15' or belwo suspect a clogged converter/exaust system. You really should check fuel pressure with a gauge. Yours should be 56-62 psi
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Tuesday, October 9th, 2012 AT 4:46 PM
Tiny
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Fuel pressure is 60-62psi under load. At idle maybe 10-15psi lower. Exhaust back pressure was measured by mechanic. He had been so convinced it was clogged converters he had already ordered replacements. He said the pressure was fine, but didn't tell me the actual pressure reading.

I'd still like to know where the ignition ground point is. It still seems like an intermittent ground. Like not enough current flows to something when I get up to speed on the highway and things heat up. If I take my foot off the pedal, it runs fine. Just a little bit of acceleration causes it to start missing.
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Friday, October 12th, 2012 AT 3:22 PM
Tiny
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Fuel pressure should be 56-62 at idle not under load, so try changing your fule filter first. Also thos eused toloosen the intake manifold bolts as they are only 8 mm. So makesure that they are tight. They sould be 30 ft. Lbs. The ignition ground is at teh right top of cylinder head it is a black w/white stripe wire and they are porbably a few there. One other thing to check is the shield around the konck sensor, some were loose and hit the knock sensor an dit retards the spark it's on eithe rside of engine by motor mount.
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Friday, October 12th, 2012 AT 4:37 PM
Tiny
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Great ideas, I'll try. I've already replaced the fuel filter twice. Once before replacing the pump, once after.
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Friday, October 12th, 2012 AT 4:54 PM

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