stumble on 87 suburban, - The 1947 - Present Chevrolet & GMC Truck Message Board Network

BIG V · 07-11-2012, 02:09 PM

Question, I have a 87 chevy suburban with the 350 and 700r4. there is a off idle stumble. i have 13 lbs of fuel pressure, the fuel filter has been swapped, I changed the plugs and wires, cap and rotor. I have removed the iac and completely cleaned the iac and the port. im stumped it does a off idle stumble acts like it has a cam in it. then it goes away the stumble only lasts for a second then goes away. Any insight would help.

Jonboy · 07-16-2012, 04:12 PM

Did you check EGR?

BIG V · 07-16-2012, 07:02 PM

yes i did. It helped the problem but hasn't fixed it.

cal30_sniper · 07-17-2012, 06:51 PM

Ignition Module and Coil are a quite common problem for TBI injected vehicles. I'd start there, replace them with AC Delco replacements, not the aftermarket "performance" offerings from Accel, MSD, etc. They are notorious for blowing out right after being put in.

If that doesn't fix it, I'd look into replacing the MAP sensor and TPS. If you have a digital voltmeter, check that the MAP sensor and TPS are both functioning properly. Also, have you checked to see if it's throwing any codes yet?

-cal30sniper

BIG V · 07-17-2012, 07:07 PM

I have a ethos to watch live data on. The tps is working great smooth waveform , Has all ac delco ignition on it, map sensor is reading correctly also. all rubber hoses have been replaced under hood, do you think if the harmonic balancer has slipped it would cause these problems since the inital timing is set on the balancer? No codes yet but you never know.

cal30_sniper · 07-17-2012, 07:26 PM

It is possible. The easiest way to check is to unplug the brown wire for ignition advance, and see how it runs on base timing. If it's set correctly and running really rough, that means your ignition is retarded. Every balancer I've seen slip "advanced itself" so that even when running the proper 0-6* of base ignition advance, the balancer would show something like 15-40* of advance at idle. If you try to retard it back to within spec, the engine will get really choppy, vacuum will drop, and it will die. As far as I know, that's the only way the balancer should ever slip due to the direction the engine is rotating.

-cal30sniper

07-11-2012, 02:09 PM	#1
BIG V Registered User Join Date: Jul 2009 Location: Northern California Posts: 411	stumble on 87 suburban, Question, I have a 87 chevy suburban with the 350 and 700r4. there is a off idle stumble. i have 13 lbs of fuel pressure, the fuel filter has been swapped, I changed the plugs and wires, cap and rotor. I have removed the iac and completely cleaned the iac and the port. im stumped it does a off idle stumble acts like it has a cam in it. then it goes away the stumble only lasts for a second then goes away. Any insight would help. __________________ 1991 Chevy suburban 3/4 ton 4x 4:56 and pulls great 1979 Chevy 1/2 ton long bed diesel 1974 Chevy crew cab short bed 3/4 ton 4x4 project 1991 Chevy suburban 2wd 1984 Chevy suburban 3/4 ton 4x 6.5 detroit with turbo 1961 Chevy pickup 1/2 ton 2wd 1975 gmc crew cab short bed 2wd

07-16-2012, 04:12 PM	#2
Jonboy Slots go on anything! Join Date: May 2003 Location: Richardson, TX Posts: 5,957	Re: stumble on 87 suburban, Did you check EGR? __________________ 1974 Jimmy- 5.3/4L80e/NP241

07-16-2012, 07:02 PM	#3
BIG V Registered User Join Date: Jul 2009 Location: Northern California Posts: 411	Re: stumble on 87 suburban, yes i did. It helped the problem but hasn't fixed it. __________________ 1991 Chevy suburban 3/4 ton 4x 4:56 and pulls great 1979 Chevy 1/2 ton long bed diesel 1974 Chevy crew cab short bed 3/4 ton 4x4 project 1991 Chevy suburban 2wd 1984 Chevy suburban 3/4 ton 4x 6.5 detroit with turbo 1961 Chevy pickup 1/2 ton 2wd 1975 gmc crew cab short bed 2wd

07-17-2012, 06:51 PM	#4
cal30_sniper Registered User Join Date: Jan 2011 Location: Coastal NC Posts: 276	Re: stumble on 87 suburban, Ignition Module and Coil are a quite common problem for TBI injected vehicles. I'd start there, replace them with AC Delco replacements, not the aftermarket "performance" offerings from Accel, MSD, etc. They are notorious for blowing out right after being put in. If that doesn't fix it, I'd look into replacing the MAP sensor and TPS. If you have a digital voltmeter, check that the MAP sensor and TPS are both functioning properly. Also, have you checked to see if it's throwing any codes yet? -cal30sniper __________________ 86 Chevrolet K20/30 Suburban - 8.1/NV4500/NP205/Dana 60/14 bolt FF, build in progress 73 Formula - 400/Doug Nash 4+1, resto on hold 86 Chevrolet K30 3+3 - 350/TH400/NP241, Air Force/Forest Service Rescue Truck, for sale 01 Ram 2500 - 5.9L Magnum, daily driver 91 BMW 325i - Chumpcar series racecar Last edited by cal30_sniper; 07-17-2012 at 06:56 PM.

07-17-2012, 07:07 PM	#5
BIG V Registered User Join Date: Jul 2009 Location: Northern California Posts: 411	Re: stumble on 87 suburban, I have a ethos to watch live data on. The tps is working great smooth waveform , Has all ac delco ignition on it, map sensor is reading correctly also. all rubber hoses have been replaced under hood, do you think if the harmonic balancer has slipped it would cause these problems since the inital timing is set on the balancer? No codes yet but you never know. __________________ 1991 Chevy suburban 3/4 ton 4x 4:56 and pulls great 1979 Chevy 1/2 ton long bed diesel 1974 Chevy crew cab short bed 3/4 ton 4x4 project 1991 Chevy suburban 2wd 1984 Chevy suburban 3/4 ton 4x 6.5 detroit with turbo 1961 Chevy pickup 1/2 ton 2wd 1975 gmc crew cab short bed 2wd

07-17-2012, 07:26 PM	#6
cal30_sniper Registered User Join Date: Jan 2011 Location: Coastal NC Posts: 276	Re: stumble on 87 suburban, It is possible. The easiest way to check is to unplug the brown wire for ignition advance, and see how it runs on base timing. If it's set correctly and running really rough, that means your ignition is retarded. Every balancer I've seen slip "advanced itself" so that even when running the proper 0-6* of base ignition advance, the balancer would show something like 15-40* of advance at idle. If you try to retard it back to within spec, the engine will get really choppy, vacuum will drop, and it will die. As far as I know, that's the only way the balancer should ever slip due to the direction the engine is rotating. -cal30sniper __________________ 86 Chevrolet K20/30 Suburban - 8.1/NV4500/NP205/Dana 60/14 bolt FF, build in progress 73 Formula - 400/Doug Nash 4+1, resto on hold 86 Chevrolet K30 3+3 - 350/TH400/NP241, Air Force/Forest Service Rescue Truck, for sale 01 Ram 2500 - 5.9L Magnum, daily driver 91 BMW 325i - Chumpcar series racecar

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