Mechanics

MY K2500 CHEVY 7.4L RUNS FINE COLD BUT WITHIN 5-10 MILES STARTS TO FALTER.

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).
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Dmccrack
October 5, 2012.




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


Tiny
Hmac300
Oct 5, 2012.
Thanks for the reply. I'll have to borrow or buy a tester that I can read while driving.
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Tiny
Dmccrack
Oct 7, 2012.
Everything can be done with just turning thekey on not driving


Tiny
Hmac300
Oct 7, 2012.
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?


Tiny
Dmccrack
Oct 9, 2012.
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


Tiny
Hmac300
Oct 9, 2012.
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.


Tiny
Dmccrack
Oct 12, 2012.
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.


Tiny
Hmac300
Oct 12, 2012.
Great ideas, I'll try. I've already replaced the fuel filter twice. Once before replacing the pump, once after.


Tiny
Dmccrack
Oct 12, 2012.

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