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ECM issues

I am trying to build a camper van and the first thing I did was rebuild the 5.0. Virtually every thing is new, but I am having issues getting the ecm programmed.
I was having issues with misfires and after much troubleshooting bought a refurbished ECM from Advanced Auto parts. Put it in and the misfires went away. Let it idle for 10 minutes not a single hiccup. Put it in gear and gave it some gas and it felt like the tranny was being ripped out and dies. Took it to the dealership, and after several tries at a new flash, they fixed the issue with the tranny. Drive it home and the misfires are back. How can I get this thing programmed right where both tranny and firing are working correctly?

Year? Mileage? Did it have the problem before a rebuild? Is the chek engine light on? Codes?

1996 GMC Savana with a 5.0. 215k miles less than 1k on motor rebuild. Misfire code, PO308.

The sensors are all new, as is cap, rotor plugs etc. Almost nothing left that hasn’t been replaced. Good compression, oil pressure 60lbs, no valves sticking. Fresh start no misfires for about 10 seconds then they begin, but they aren’t especially bad.
The key to this puzzle, I think, is that the computer fixed the misfire, but when programmed from the dealer it returned. Seems there has to be something going on with the ECM right?

Surely seems to point to a bad ECU. Replacing it again seems to be best option.

I have tried 4 junk yard computers and 2 from the parts store with similar results. The only thing I have not tried is getting a unit from a 5.0. Everyone says the 5.7 works the same, but it hasn’t. Tried one from a 4.3, 3 from a 5.7. Hard to find used ones for a 5.0

Curious, do you think it could be a faulty wire to my O2 sensors? Putting a motor in a van is tight, something could have been smashed or pinched. If so, how do I test this?
Thanks

When cold the engine runs open loop because the O2 sensors won’t read correctly until warm. So, the wires could be damaged but that shouldn’t keep the truck from running.

They can be checked with an ohm meter or test light from the ECU header connector to the O2 sensor pins.

Since it IS running, but with misfires, I’d be checking the engine harness wiring. I’d swap the #8 plug and wire with another of similar length just to make sure that the replacement part isn’t bad. If the P0308 moves to the swapped cylinder, that’s a clue.

I just checked Rockauto… this van has the injector spider system. Has THAT piece of crud been replaced with the revised spider that has 8 actual injectors?? Your van’s problem defies logic and that injection system and its problems defy logic, too.

Has it been replaced with this?

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The spider injector is new, but it doesn’t look identical to that one. Not sure what brand was purchased. I’m the owner but no mechanic, so I am just trying to fumble through this diagnostics best I can.

Going to have the fuel trim checked this morning and go from there. Mostly just ruling things out that might mess with the ECM.

So here is a weird anomaly. Pulled the connector off the MAP sensor, and the misfire subsided. Put a new MAP sensor on the manifold thinking that would resolve this but the misfire returns. I’m good with the map off, as soon as it’s connected, misfires return.

In the abscence of the MAF, the ECU does its best to estimate the volume of air. Check the air hoses between the MAF and the engine. If there is a leak, that can cause the engine to run lean. Lean can cause misfires.

All through this… do you own a code reader or have a reader program on your phone? If not, you are working completely blind and just swapping parts hoping one will fix it. That costs you more in parts than a $15 bluetooth odb II reader and a free phone app. You can read in real time a lot of useful things that will help diagnose this.

My brother is doing the work and he has a snap-on solus ultra.

Are we talking about a map sensor or a maf sensor? You said MAP sensor, but @Mustangman is talking about a MAF sensor.

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Map sensor

Ok. Hook up a vacuum gauge to the hose that connects to the map sensor and see what reading you get at idle and at about 1800 rpm. Also note if the needle is steady at idle.

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