Rpm drop when stopping.

[Hodgeee]

That makes me wonder if the new pump was not right. It sits at 48psi, but dips to 43 right at that rpm dip. According to my GM service and repair manual 43 to 48 is listed as normal pressure levels. The stock pump ran right at 43 and never dipped. But it would jump into the mid 55 range when hot, just like the new pump does. I also swapped a new fuel pump control module since the first dealership told me thats what it might need. My friend programmed it to my truck. No different. I usually wait till the pump goes out before swapping them, but ive done 4 chevy truck fuel pumps, and the longest mileage one was 180,000 miles. Probably the Vegas heat. So since i was refreshing this truck for long hunting, fishing, and camping trips, i just did it all at once, trying to prevent possible breakdowns. I dont like throwing parts at a fix, but 2 dealerships, and 4 chevy mechanics saying they dont know, try this or that doesnt help. Im done. Everything has been replaced or tested. If the new GM TB doesnt fix it, i dont care anymore, its fine, love the new to me tahoe. Its my third. 280,000 miles on my first two, 10 years on each.

 

[swathdiver]

That makes me wonder if the new pump was not right. It sits at 48psi, but dips to 43 right at that rpm dip. According to my GM service and repair manual 43 to 48 is listed as normal pressure levels. The stock pump ran right at 43 and never dipped. But it would jump into the mid 55 range when hot, just like the new pump does. I also swapped a new fuel pump control module since the first dealership told me thats what it might need. My friend programmed it to my truck. No different. I usually wait till the pump goes out before swapping them, but ive done 4 chevy truck fuel pumps, and the longest mileage one was 180,000 miles. Probably the Vegas heat. So since i was refreshing this truck for long hunting, fishing, and camping trips, i just did it all at once, trying to prevent possible breakdowns. I dont like throwing parts at a fix, but 2 dealerships, and 4 chevy mechanics saying they dont know, try this or that doesnt help. Im done. Everything has been replaced or tested. If the new GM TB doesnt fix it, i dont care anymore, its fine, love the new to me tahoe. Its my third. 280,000 miles on my first two, 10 years on each.

I've never seen more than about 2 or 3 psi on either side of 43.5 on my three Gen IV LS engines. Interesting. Shop manual for mine says 43 to 45 psi.

 

[Hodgeee]

Yeah, i was surprised also, but saw it was in spec.

 

[Foggy]

My flex fuel yukon calls for 58 psi... maybe the 5.3 used a diff pump, controller or injectors

 

[Hodgeee]

Do you have the 6L?

 

[Hodgeee]

Wow, put in my new Hitachi TB(original supplier for GM throttle bodies) did the relearn. Now the rpms dip about 50rpms only on very hard breaking, which is normal. Dorman electronics suck from my experience. This was replaced by original owner 12000 miles ago. My friends Avalanch had a Dorman TB also, and his idle has always been a little erratic also.

Problem fixed, hopefully my gas mileage will get a little better also.

Thanks all for the advice and ideas.

 

[NickTransmissions]

Wow, put in my new Hitachi TB(original supplier for GM throttle bodies) did the relearn. Now the rpms dip about 50rpms only on very hard breaking, which is normal. Dorman electronics suck from my experience. This was replaced by original owner 12000 miles ago. My friends Avalanch had a Dorman TB also, and his idle has always been a little erratic also.

Problem fixed, hopefully my gas mileage will get a little better also.

Thanks all for the advice and ideas.

Great to hear, man!

Had a feeling the Dorman POS throttle body was the source of your problems...

 

[Hodgeee]

Hey guys, well it took a couple days, the rpm drop is back, and i dont know if i stated before, after almost completely stopping the idle surges and the vehicle lurches forward requiring harder breaking. Im wondering if the manual idle reset is working, or i need a quality scanner to relearn the TB. The old TB was replaced 16000 miles ago, and i womder if it was ever reprogrammed. Ive read else were only using a high end scanner will work.

So my hand held tuner i noticed the throttle input compared to the predicted throttle input are way off. I know at idle the pre should be about 8% which mine hovers between 6 to 10% , and also at high throttle inputs the pre should be higher, but under almost all other driving conditions they should match. My pre is double that the throttle input. Does anyone know if this is resetable with the TB relearn, and could this be my problem?

 

[Doubeleive]

Wow, put in my new Hitachi TB(original supplier for GM throttle bodies) did the relearn. Now the rpms dip about 50rpms only on very hard breaking, which is normal. Dorman electronics suck from my experience. This was replaced by original owner 12000 miles ago. My friends Avalanch had a Dorman TB also, and his idle has always been a little erratic also.

Problem fixed, hopefully my gas mileage will get a little better also.

Thanks all for the advice and ideas.

I was just going to say the gm throttle bodies are made by Hitachi so as long as it is Hitachi it should work. i changed mine out to a LS7 model because the original one had a little "sputter" in it is the best way to describe it. Plug n play didn't even need a relearn. I figured if it is good enough for a corvette, camaro, g8 then it should be good for me lol
my fuel pressure sit's pretty much at 43.5/44 (running)

 

[Hodgeee]

Yeah, my new AcDelco pump, fluctuates alot compared to the old pump which sat steady at the 43 mark, my new one hovers around 48psi, but dips sometimes to 43, but the brakimg idle situation has been there since i bought the truck.