Baffling Problem - Engine Revs Truck Does't Move

[Marleay]

2006 Chevy Silverado with 4.8L V8. Daughters truck.

 

She calls and says the truck is revving but hardly moving then if she turns off the engine and starts again it runs fine for a minute or two and then goes into 'limp' mode and then like it is in neutral.

 

I put a code reader on and it had some codes. I cleared them and then was left with MAF sensor issue and oil pressure sensor failed. Replaced the MAF sensor. Still have problem.

Someone suggested the ignition switch so I replaced the ignition switch module (not the actual key part). Still have problem. 

 

Transmission fluid is up, looks and smells fine.

 

It also does a strange thing when reconnecting the battery - the engine tries to start and dies on its own 3 times - scary when it happens!

 

Help please...

[Supreme Pizza]

When you reconnect the battery, it tries to start by itself 3 times in a row? That sounds like you have an OEM or aftermarket remote start system that is not working properly. You may have a bad "computer". I would not replace any more parts without having a diagnosis. Its easy to spend $500 on parts that don't fix the problem, and a diagnostic only costs $100. What ever parts you put on, take them back off and return them if they didn't solve the problem. Then take that money and pay the dealer or a GOOD local shop to do a 1 hour diagnosis - just to see what they can come up with.

Edited December 18, 2021 by Supreme Pizza

[Marleay]

Thanks.  I think it is an aftermarket auto starter and it did have an alarm system installed as well. We don't use either of those but there is some wiring under the dash associated with those systems.

The truck was one my company trucks that got passed on to the kids - so had it for a long time, almost since new. It was running rough and some of the original codes were to do with tht  The new MAF sensor fixed that.

 

My guess is that it is the main ECU but I have a diagnostic code reader 

[Marleay]

Having trouble typing replies on this site... really bad lag.

 

I have a good diagnostic code reader with Bluetooth connection to iPhone app. No codes from any modules. I have the oil pressure sensor which I will change out but it is in a hard to reach place. I can't see that causing the problem but the gauge doesn't work so it is worth replacing it.

[Supreme Pizza]

If your oil sender is like mine, then you need a large socket, a swivel, a ratchet, and some extensions. A mirror and flashlight help too. Its worth wearing light gloves and long sleeves to keep from getting scratched up when your reaching back there.

 

Its easier to change the oil sender if you pull the intake, which is not hard at all - but it is probably more time consuming to do it that way.

[Marleay]

43 minutes ago, Supreme Pizza said:

Its worth wearing light gloves and long sleeves to keep from getting scratched up when your reaching back there.

Thanks for the late advice...I was feeling around in the area a couple of days ago and now have scratches from knuckles to half way up my forearm! 

I have all the tools and the 1-1/8" deep socket but waiting to change the sensor.  I need to get the running problem figured first.

Ive had/have lots of GM vehicles and never experienced a 'limp mode' with any of them.

 

My only experience with that mode was in my Mercedes Sprinter van RV with turbo diesel going over the Coquihalla highway pass coming out of Vancouver. Minor issue but a big tow bill!

 

[Supreme Pizza]

For the limp mode, since its accompanied by an oil pressure issue, I would normally say that it could be an AFM issue - but I do not believe that your year/ engine size came with AFM as an option. However, since the gauge is not reading anything, are you sure that you are in fact building oil pressure? Also, what happens if you drop it into the 4x4 low range? Is it able to drive around in that mode?

[Marleay]

The oil gauge is pegged at high. Diagnostic said the oil pressure sensor was "shorted".

I've tried the drivetrain in forward, reverse, 3,2,1, 4WH and 4WL. I thought maybe the torque converter but that was eliminated from the driving tests.

I'm not a mechanic but grew up fixing what I drove.(1963 Ford Fairlane 2 door Sports Coupe was my first car) Dad was a petrolhead in England and would build full cars from bits and pieces after the war. I have an electrical contracting company now.

 

Jack of all trades...master of some!

[Marleay]

What surprises me is that I've talked to a mechanic and a transmission rebuilder and now posted here and nobody seems to have heard or seen anything similar happening to a GM truck.

The exception being a brother-in-law who had similar happen to him and the fix was the ignition switch.

 

[Sharpz]

ok sorry I didnt see this post earlier, or read it but, I see you say you changed the maf.  Why didnt you change the oil pressure sensor?  If it is bad, sometimes your truck will think the oil pressure is too low, and will cause damage so will shut down, in your case go to limp mode.  When you change the Oil pressure sensor also test the plug, to make sure you have a good connection.   

 

for a 5.3 the socket size is a 36mm and you will need a 3/8 ratchet extension 4 to 6 inch and also remove the plastic tb cover 4 bolts I think are 8mm. This is done by feel, but there are plenty of you tube videos on this.

 

Edited December 19, 2021 by Sharpz

[Supreme Pizza]

Originally, I was thinking about the oil pressure issues shutting down the engine, but I have seen these motors run with 0 oil pressure for 15 seconds, with no automatic shut down. Not just a oil sender problem, but actually 0 pressure in the system. They just had warning on the dash. Is there a built in safety that will turn the engine off or at least some of the cylinders?

 

 

[Marleay]

2 hours ago, Sharpz said:

Why didnt you change the oil pressure sensor?  If it is bad, sometimes your truck will think the oil pressure is too low, and will cause damage so will shut down, in your case go to limp mode.

I haven't changed the oil sensor yet. It is in a bit of bad place on the 4.8L engine so I'm putting that off to last. 

I have all the fluids to do the transmission and both front/back differentials now. will try to get to that in next 2 days.

 

I don't drive the truck so I don't know when the sensor/gauge started acting up. it is an input to the engine management system so maybe there is an output that causes these symptoms. 

 

The baffling part is the engine revs to redline without an issue during this problem. I would have thought the ECU would have limited the engine revs in 'limp' mode and let the vehicle limp home.

[Supreme Pizza]

Lets go over the symptoms again, if that's cool with you. There was some "he said she said" 3rd party stuff in the original post.

 

What are the exact symptoms, codes, and things you have already tried?

 

 

 

 

Edited December 20, 2021 by Supreme Pizza

[Marleay]

Okay, totally understand that. I think it comes down to 2 separate problems.

 

In the past few months the truck was running rough. 

Pulling up to a stop the revs would go down and then surge causing the truck to move forward.

Daughter complained but nothing was addressed at the time. 

Service engine light was on.

 

Last week she phones to say that the truck starts and runs but then won't move when in gear.

I have her bring it to the shop and put a diagnostic reader on and clear codes.

Codes were symptomatic type i.e. intake, combustion, exhaust out of range.

Cleared codes. Test drove it. All drive fine for 2 minutes or so and then acts like trans in neutral.

 

Codes that remain are MAF sensor and oil pressure sensor (shorted).

Replaced MAF sensor. Put injector cleaner in fuel.

Engine runs fine but after 2minutes it is all revs and no go.

Shut engine off. Start again. Runs and accelerates through the gears ok for about 2 minutes.

Repeats the same sequence over and over again.

 

So MAF was one problem which I think is fixed. Engine starts and runs smooth now.

 

Lack of motion after a couple minutes is the baffling conundrum now.

 

[Marleay]

At this point there no codes except the oil sensor failure.

All other modules scanned come up fine.