1987 GMC S15 Engine backfires under a load

JESSESHAFFER

MEMBER
1987 GMC S15
2.5L
4 CYL
2WD
MANUAL
188,000 MILES

My engine is running rough at an idle, I can smell gas fumes all the time. It backfires while under a load on acceleration. I've changed the fuel pump and filter. Changed plugs, wires, cap, rotor, and coil. Changed air filter, it is only a week old and is black on the inside from the flames coming out of the throttle body. I've changed the O2 sensor also the EGR valve. Plugs are carboned up after only a few days. Engine doesn't use oil or smoke. What else could be my problem. I've also changed the vacuum lines. Thanks for the help.

Thursday, June 11th, 2015 AT 1:42 PM

7 Replies

JDL

MECHANIC
16,098 POSTS

If it backfires out the throttle body, check the intake valves and valve timing. Check for proper fuel pressure, with tbi, around 9--13 psi.

Block the fuel supply line and return, see if pressure will hold for a bit. Smell of your crankcase oil dipstick, see if it smells like gas. Sometimes where applicable, your evap system can leak and you smell it.

Can you check for any applicable trouble codes? If your check engine lite still works, you should be able to check some codes, yourself.

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Thursday, June 11th, 2015 AT 4:24 PM

JESSESHAFFER

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8 POSTS

Its not throwing any codes at this time. I placed a dollar bill over the tail pipe and it tries to suck it in. Would that be carboned up intake valves? Should I replace my head with a rebuilt head? And if I replace the head, what is the procedure for valve adjustment and all the torque specks and procedures?

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Friday, June 12th, 2015 AT 5:23 AM

JDL

MECHANIC
16,098 POSTS

Intake and exhaust valve specs.

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Wednesday, June 24th, 2015 AT 9:03 AM

JDL

MECHANIC
16,098 POSTS

Just to add, torque specs, cylinder head.

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Wednesday, June 24th, 2015 AT 9:07 AM

JDL

MECHANIC
16,098 POSTS

Just to add. This is valve arrangement for intake and exhaust. As far as lifter adjustment, hydraulic lifters, You loosen the adjusting nut till you can move push-rod up and down, then slowing tighten adjusting nut until no more play.

With mechanical lifters you use a feeler gage to adjust.

I can't tell if cylinder head needs replacement? Any machine shops in your area? They could probably give your cylinder head the once over and give recommendations. I can't see much from here.

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