Engine stalls for 1 second when decelerating in overdrive.

rumblebee · December 24, 2015

I'm a newbee here, and really need your thoughts on a strange problem.

I have a '99 Suburban LS 4x4. The 5.7 was rebuilt 6 months ago by a shop that was highly recommended, but I've had nothing but trouble since. I won't get into all the problems I've had, but I need help with this one: from day 1 after the rebuild, at 45 mph and above in OD, if you let off the gas there's a brief clunk (the tranny doesn't slip). This only happens after the engine is warms up (runs fine cold).

I took it to a mechanic that I trust completely, and he drove it with his scanner connected - when it clunks the engine is actually shutting off for a second (or less), then comes back on. You can't see this on the tach, but he's seeing it on the scanner. In the parking lot, he power-braked it up to 1500 rpm, then let off the gas and it immediately stalled. No codes are showing. The truck always ran great & shifted smoothly before the rebuild.

The motor was bored .030 over, flat-top pistons 10:1, and a mild roller cam installed.

Since the rebuild I put in a new distributor, cleaned the throttle body, and replaced the TPS & IAC valve. The plugs & wires are a year old.

I'm sure that my new mechanic will find the problem, but if anyone has ever had this problem or has any ideas I'd really appreciate you input.

Squirrely · December 24, 2015

did you reprogram for your upgrades?

rumblebee · December 25, 2015

did you reprogram for your upgrades?

No - still the stock programming. According to Comp Cams, the cam doesn't require re-programming, but do you think that the higher compression in combination with the cam would mess with the computer?

Thanks, and Merry Christmas!

txab · December 25, 2015

I would get a tune for the changes/upgrades

Squirrely · December 25, 2015

Yes I do. You've changed lift, duration, etc. The engine is not programmed to run that cam... no matter what any cam company says. You might need more fuel pressure too. Maybe..

Squirrely · December 25, 2015

Even changing tire diameter can sometimes cause issues with programming. I've seen it on a F250 before. Ran like junk and poor shifting.

rumblebee · December 25, 2015

Thanks guys. I guess I'll have to have it reprogrammed. I put 18" Z71 rims with Goodyear P265/65Rs on it too - so I unknowingly threw another wrench into the equation... I've never dealt with mods on a computerized engine before, so this is all new to me. I'm also electronically challenged; my 3 year-old granddaughter can do more with my "smart" phone than I can. Life and Chevy engines were much simpler in the 60's & 70's!!

So how do I go about getting it reprogrammed, and who can do this with the modifications?

Thanks for your help.

rumblebee · December 31, 2015

Update: The mechanic found the problem; the spider injector tubes were in the wrong holes... All of them! I'm glad I found a great mechanic - how many people would even check that on a fresh rebuild?

The truck runs good now and the OD clunk is gone.