Runs fine-then it doesn't-then it does

[Paul Bell]

My engine starts fine and runs fine for a good while then it drops down to a very low idle-like 200~300 RPM and won't come up when stepping on the throttle. Shut it down and it wants to fire but as soon as it does it dies. Wait a while like 20 minutes and it starts right up and runs fine again. Drive five minutes and it does it again!

Bad fuel pump, right? It had to be, it had 210,000 miles on it. I order the ACDelco unit and arrange it's installation. Good thing too, the nipples coming out of the pump were rotted. How it wasn't spraying fuel is beyond me. The nipples broke off in the nylon lines and I had to get them from GM. Costly!

THE PROBLEM STILL EXISTS. It had all the signs of a bad fuel pump I didn't bother checking the pressure.

Now I have. Good pressure at all times, cranking, running and while the problem exists.

Check engine light is on but I figured it was because there's transmission codes-third gear is dead and I'm about to R&R the unit. I'll be scanning codes in the AM but meanwhile:

Has anybody experienced this before?

 

[The_Roadie]

Ever cleaned the throttle body? How old are the plugs and O2 sensors?

 

[Paul Bell]

The spark plugs are recent, the throttle body is clean and bone-dry.

The o2 sensors are no doubt very old.

I cannot imagine the o2 sensors are causing this, at worst the engine would stay in open loop but still run decently.

 

[Paul Bell]

Here's the codes:

P0101: Mass airflow sensor performance.

P1516: Throttle Actuator position performance.

P0641: 5 volt reference circuit 1

I have the GM factory repair guide. This 5 volt circuit feeds the MAF, TAC and the OPSU. I've been having issues with the OPSU going zero OR max. One of the repair actions is to replace the OPSU. I'm leaning towards this being it. If it grounds the 5 volt side, the other sensors become disabled. I have a new ACDelco Professional one in a bag, from the Chevy dealer counter.

It could also be a wiring issue such as a grounded wire in the harness or worse, a bad PCM. HOWEVER, the fact that the problem is intermittent leads me to suspect the OPSU.

Too bad it's hailing outside right now...

 

[Paul Bell]

OK, here's the verdict:

I replaced what is obviously not an ACDelco OPSU with the genuine GM article. Aftermarket unit was brass, OEM is aluminum.

When I started the engine, the oil gauge read normal, around 40~45 PSI instead of the zero or maxed it was before.

Confirmed, the OPSU was killing the 5 volt reference that also powers the MAF and TAC.

A real head scratcher but it's done.

 

[BlazingTrails]

Glad to hear it man! electrical problems especially sub 12 volts dc are very hard to track down sometimes.

 

[Paul Bell]

Years ago I had a girlfriend with a Toyota Corola. It liked to overheat. The radiator got so hot one time, the fill/cap neck became unsoldered.

It was a bad fuel pump. It was causing it to run lean.

 

[Mounce]

 

[IllogicTC]

OK, here's the verdict:

I replaced what is obviously not an ACDelco OPSU with the genuine GM article. Aftermarket unit was brass, OEM is aluminum.

When I started the engine, the oil gauge read normal, around 40~45 PSI instead of the zero or maxed it was before.

Confirmed, the OPSU was killing the 5 volt reference that also powers the MAF and TAC.

A real head scratcher but it's done.

I wonder if it's possible to install the I6 oil pressure switch in the V8 by accident -- if it's possible, it's likely someone gave the wrong information, or someone else the wrong part, at AutoZone.

 

[Paul Bell]

I got the unit from Amazon. It was a Standard Motors product, it was the correct PS308.

My brother worked there decades ago, he designed Jiffy Kits (carb rebuild kits) and HEI ignition stuff. It used to be the best you could buy.