1987 350 TBI running rich at idle with occasional backfire

[Shoote]

Does anyone know about 350 TBI engines? I have a 1987 V10 with the original 350 TBI. The truck runs well above idle, never stalls at idle but idles rough. That is the positives. Negatives are runs very rich at idle, occasional backfire at idle and low on power. I have replaced the MAP sensor, IAC, temperature coolant sensor, new distributor with ICM, full tune up, wires and plugs. Pulled the tan with black stripe wire, set the timing to 0 degrees. Motor ran even worse so I set timing to 8 degrees BTC. Small improvement. Checked EGR, it works fine, vacuum checked lines going to EGR. Each line held over 20 on the vacuum guage so no leaks there. Truck shows codes of 42 and 32. Any help is appreciated!!!! At this point.....one small step away from a Blueprint 383 repalcement.

 

[AuroraGirl]

Does anyone know about 350 TBI engines? I have a 1987 V10 with the original 350 TBI. The truck runs well above idle, never stalls at idle but idles rough. That is the positives. Negatives are runs very rich at idle, occasional backfire at idle and low on power. I have replaced the MAP sensor, IAC, temperature coolant sensor, new distributor with ICM, full tune up, wires and plugs. Pulled the tan with black stripe wire, set the timing to 0 degrees. Motor ran even worse so I set timing to 8 degrees BTC. Small improvement. Checked EGR, it works fine, vacuum checked lines going to EGR. Each line held over 20 on the vacuum guage so no leaks there. Truck shows codes of 42 and 32. Any help is appreciated!!!! At this point.....one small step away from a Blueprint 383 repalcement.

sounds like maybe the timing chain or stuff has some wear or the valves aqre sticking.
pop your valve cover what you got under it

 

[Shoote]

Thanks for the reply. Clean under the valve covers. I have driven this truck on two 1200 mile round trips in the last 4 months. Runs great off idle aside from low on power.

 

[gmbellew]

code 42 is the EST. could be a bad new ICM going out or potentially a wiring issue if the code isnt a remnant from when you unplugged the EST to set the timing. I'd hit up the drivability manual for your year in the reference section. 8 degrees is pretty advanced base timing...could be something else mechanical going on to require that much base timing to run better.

 

[mtbadbob]

Did you rebuild the TB? The regulators leak and make it hard to start and run rich at idle as they flood the engine. Also check or replace your O2 sensor.

 

[AuroraGirl]

You must be registered for see images attach

old vs new if wanted to see an old, but not bad one

 

[Shoote]

Thanks for the responses. I did replace the 02 sensor. Throttle body has been rebuilt. Injectors are not leaking. I am out of ideas.

 

[AuroraGirl]

Thanks for the responses. I did replace the 02 sensor. Throttle body has been rebuilt. Injectors are not leaking. I am out of ideas.

Find your ECM. lightly tap the ECM while it idles.
Report findings.

Then, if it still issue, remove battery power, remove pcm from holder and harnesses. clean connector. and ecm. and pull the PROM out carefully and reseat it and then put it back in and check.

 

[eric 87]

Well well well. Reading yours is just confirmation I need to do the swap I am on the computer to buy the parts for. I figure at least these parts I can use on a new engine if I should decide to swap it. My engine is strong enough and runs good SOMETIMES. Just wait yours may end up quiting as you drive down the road at a steady speed. Losing all power briefly.
I have an 87 which is the first year of the computer. Seems that everyone of these trucks have this issue at some point. I have replaced EVERY SINGLE thing. Including the alternator. I replaced the computer, (2) temp sensor, map sensor, IAC (2) , O2 sensor, distributor wires and plugs and now the Ignition module. After all of this replacement I am now getting the code 42 as well. Since I replaced the BRAND NEW ignition module that came in the new distributor I have come to the conclusion it must be a short in the plug/ wiring from the ECM to Ignition module per another forum. I have a diagnosis tree if you would like on how to check those wires.
OH and I replace both fuel relays on the firewall.
All these parts will soon be for sale as I am going with a carb conversion. Ordering parts today. Once I get it running correctly...which it does often and the parts did make a difference in performance and mileage.....I will either sell the truck or change the wiring harness and also donate to someone elses headaches including the computer. Then I will swap engines if needed at some point. At least the carb, intake and distributor would work on a long block.
My advice for any future square body owners is to not buy an 87 TBI. As someone else said. They may have really been good at the time but since each and every circuit is connected once you have a wiring issue it effects every other thing in the chain. Go back one year and.....
Funny I am basically changing my 87 into a carb 86!

 

[AuroraGirl]

Code 42 is EST and Im pretty sure there was a TSB for a distributor faulty parts thing for it I can check for you if youd like

 

[Shoote]

Thank you for the replies. Turns out it is a burnt valve. TBI is going away!!

 

[WP29P4A]

Thank you for the replies. Turns out it is a burnt valve. TBI is going away!!

I'm just curious for future trouble shooting, how did you figure out it is a burnt valve? Will a burnt valve show up on a compression test? Is there problems with the TBI also?

 

[Bextreme04]

I'm just curious for future trouble shooting, how did you figure out it is a burnt valve? Will a burnt valve show up on a compression test? Is there problems with the TBI also?

Hopefully it was found the right way by doing a compression test and then a leakdown test. You would know you need to check for a mechanical issue by using a vacuum gauge with the engine running. Bouncing vacuum will tell you that you either have a valve issue or broken rings. Compression test will give you which cylinders are suspect. Leakdown test will tell you exactly what the issue is. You'll hear air leaking either through the intake, exhaust, or breather depending on which part is bad in a given cylinder.

 

[fast 99]

A burnt valve would cause low vacuum. The MAP sensor would read low and increase fuel flow "thinking" the engine was under a higher load. Result, rich condition.