1990 GMC C1500 Loping Idle and Surging Acceleration

XJOHNDOE001X

MEMBER
1990 GMC C1500
300,000 MILES

Friends, I am desperate, I tried everything I know and more but I cannot find a solution to my Loping idle and surging acceleration on my '90 Sierra C1500, 5.7L TBI, I've replaced the Timing Chain & Gears, IAC valve, TPS, MAP, EGR & Solenoid, ES Module & Knock Sensor, ECS, Ignition Module, Plugs & Wires, Dist. Cap & Rotor & Fuel Filter.

Compression is 118psi to 120psi and fuel pressure 10psi, within the 9 to 13psi tolerance. I reset the IAC Valve by jumping the A & B terminals on the diag. Connector, * turn to ignition to the on position, NOT START, wait a minute then disconnect the IAC Valve electrical connector. Then turn the ignition to the off position, remove the jumper from the diag. Reconnect the IAC connector. Then start the engine and set the idle speed. (Method from Chilton Manuel, different than ThirdGen article method which I also tried.)

My COs is 160 but should be 20, my HCs is 2.95 but should be 2.00. Nox was 0.34 well below the exceptible 4.00

My Injectors spray pattern seem right though the droplets size seems larger than I imagined, but uniform and smooth. The only relivant parts that I haven't yet replaced is the Injectors, Ignition Coil, Ignition Pickup Coil.

I'm near my wits end, please help this desperate man regain some sanity.
Sincerely xJohn Doex

6 Replies

HMAC300

MECHANIC
48,601 POSTS

Spray choke cleaner around the tbi unit at bottom gasket blows common failure. Check for soft hose going to map sensor sucks together and map won't work.

Was this

answer

helpful?

Yes

-1

Monday, August 5th, 2013 AT 8:10 AM

XJOHNDOE001X

MEMBER
4 POSTS

Thank you much for your quick response, but please, allow me to restate what you said to be sure I understand:
(1) I should spray brake cleaner around the base of TBI to check for vacuum leak because TBI gasket failure is common?

(2) Check the hose from manifold to MAP sensor for signs of collapsing?
The TBI gasket is new, However, the manifold is aluminum and I may have let my fear of stripping the bolt hole threads cause me to not tighten the TBI bolts enough. That may be why it's a common failure.

SPONSORED LINKS

Was this

answer

helpful?

Yes

-1

Monday, August 5th, 2013 AT 12:44 PM

HMAC300

MECHANIC
48,601 POSTS

Chokecleaner not brake cleaner. Torque should be no more than 15 ftl bs. Try seeing if it is at that torque if not torque to that and see if it cures it.

Was this

answer

helpful?

Yes

Monday, August 5th, 2013 AT 1:01 PM

XJOHNDOE001X

MEMBER
4 POSTS

Somewhere here on 2CarPros it was said that exhaust back pressure will cause loping, is this section of exhaust pipe that is dropped down a couple pipe sizes as seen in the attached pic enough to cause back pressure?

Image (Click to make bigger)

Was this

answer

helpful?

Yes

Monday, August 5th, 2013 AT 10:10 PM

HMAC300

MECHANIC
48,601 POSTS

It will cause a little but not enough to make it lope I think. Have you scanned for codes all youneed is apaper clip go between A&B on the aldl and count flashes of service engine light. That should let you know a bout a sensor. Some of those tbi units ahd poor connections at top but I dont' think that is it it may be the fuel pressure regulator on tbi unit that is causing the problem. I really can' tthink of any thing else. If you have 15 psi of fuel it shodl only be 9-13 at most so it lmay be the regulator or a blocked return line