Hello, I have a summit crate engine part # 150-153 in my 1986 Jeep J20 with a 4l60e and np241C that I am having issues running correctly. The swap runs and drives fairly well but I have had issues getting it tuned correctly since the swap.

Details:
5.3L summit 150-153 crate engine
10:1 compression
Lift- 550/550, Duration @.050- 222/232
799 Heads
2006 P59 PCM
DBW throttle body
Factory 2006 truck, coils, injectors, exhaust manifolds, accessories
Mechanical fan
No emissions equipment
No cats, O2 sensors just past the manifolds

The issue I am having is the truck seems to run lean and often stumbles under load at low RPM right when moving from a stop. It also does not seem to have very good power at low RPM. At high RPM it does pretty well. I have had this truck dyno tuned twice, the first being in Texas at true street motorsports of which was awful. After they were done the truck always idled at high RPM and had a tendency to stall when the AC kicked on. I have since moved and last had the truck tuned in Utah in the SLC area but also had poor results. The truck does idle well now and doesn't stall but I still have a stumble issue and believe the truck to be running lean. The last tuner claimed there was some fuel system issues but the pressure at the rail maintains above 58 psi at all times. He also claimed the pcm would pull timing at random spots but I cant find out what would cause that. I have dug through the truck and can't identify any vacuum leaks and all sensors look to be working correctly according to my logger.

I am looking for some help identifying what may be causing this before going back to another tuner as I am getting tired of spending several hundred dollars for no results. I don't have HP tuners but do have OBDX pro for logging and have attached some logs from today. I have also have a bench harness and tuner pro and may be able to provide a tune file with some assistance.

I appreciate any assistance.

 

@SJohn,

We appreciate you going with the SUM-150153 5.3 long block. We get the frustration, especially after putting time and money into a couple tunes and still chasing the same issue.

This uses our stage 3 truck cam SUM-8713R1. At 112+5 with 3* overlap, it’s not a “big” cam, but it's enough to introduce some reversion and it will usually need a solid tune to behave well down low.

The second log helps, but it’s still mostly idle. What stands out is both banks are positive on STFT the whole time, and Bank 2 is consistently higher. So it’s adding fuel, but that doesn’t necessarily mean it’s actually lean in the cylinder.

GM does run the upstream sensors pretty close to the manifolds from the factory, but that’s with a stock cam, cats in place, and very stable exhaust flow. Once you add more overlap, remove the cats, and change the exhaust system, you can start to see some reversion at low RPM that the stock setup never had to deal with. That can cause the sensors to see extra oxygen and report lean, so the PCM may add fuel and the tune can end up chasing that. It’s not just the location by itself, it’s the combo.

Since it runs better up top, we would probably start by looking more at the low airflow side of the tune. VE and MAF down low, transient fueling, and timing. If it's pulling timing in those areas, it can make the engine feel lazy regardless of AFR.

Fuel pressure being steady is good, but it doesn’t necessarily rule out injector data or airflow modeling being off.

We wouldn’t chase a single fix here. This looks more like a combination issue in the low RPM and tip-in area than one obvious problem.

 

Thanks for the reply! Is the difference between bank 1 & 2 short term fuel trims large enough to be concerned about?

 

What throttle body and TPS are you running?

 

2006 GM Truck DBW Throttle body, PSI Conversion C10 Pedal https://www.psiconversion.com/produc.../ACC-1049.html

 

Make sure to use GM sensors as some aftermarket sensors seem a bit wonky. GM TB should be good as long as it's compatible with the computer.

 

PCM, wire harness, sensors, accessories, etc are from a 2006 chevy 1500 since I originally did the swap with a donor truck. Original motor had an oiling issue and ate the internals so then I put in the crate engine and moved everything over.

 

It’s not a large enough difference to be a main concern on its own. A few percent bank to bank is pretty normal. It’s just something worth noting, but not something we’d chase by itself.

 

Thanks for the replies. I decided to go ahead and get a MPVI4 so I can look at the tune and possibly learn how to edit things myself. I went ahead and did a read and have attached the current tune.