Stumbling, hesitating esp under acceleration
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BruceHislop
Jun 2, 2024, 11:32:08 AMJun 2
to GMC Motorhome EFI Group
Hi All,
I helped a GMCer (Greg Lipsett) install a new EFI wiring harness into his coach, along with an upgrade to Electronic Spark Control. He took it for a test run alone of 10 minutes or so and he said it ran well. Then I went with him and it started stumbling, missing and bogging especially upon acceleration.
Some background:
This was previously Dan and Terry Gregg's coach. It had an early Holley EFI system, somewhat like a Howell system, but with a Holley control module. When the obsolete control module failed in it, I helped him install an EBL equipped 7747 ECM. It ran well for several years, but the wiring harness was in very bad shape with lots of splices. This spring he purchased a Speedway EFI wiring harness plus an electronic controlled distributor and knock sensor & filter.
Normally I like to make only one change at a time, test it for proper operation, then make the next change. But in this case to keep the mechanical HEI distributor would need to wire up the 12 to 5 volt buffer circuit board into the new wiring harness between the existing distributor and the EBL. I decided to skip directly to adding the ESC distributor at the same time. Consequently I don't know if its a fuel problem or an issue with the new (used) ESC distributor. At first it seemed to run fine until the engine was hot, but now my understanding from Greg is it stumbles from cold.
The symptoms are somewhat like the issues I had when I installed my Howell system and Holley had reduced the size of the injectors from 80 down to 63pph. This Holley TB has the 80pph rounded top injectors. There is a fuel pressure gauge on the dash which shows it is running with normal fuel pressure. If it was a block fuel filter etc, I would expect to see the fuel pressure drop, but it stays steady.
So far we have:
I helped a GMCer (Greg Lipsett) install a new EFI wiring harness into his coach, along with an upgrade to Electronic Spark Control. He took it for a test run alone of 10 minutes or so and he said it ran well. Then I went with him and it started stumbling, missing and bogging especially upon acceleration.
Some background:
This was previously Dan and Terry Gregg's coach. It had an early Holley EFI system, somewhat like a Howell system, but with a Holley control module. When the obsolete control module failed in it, I helped him install an EBL equipped 7747 ECM. It ran well for several years, but the wiring harness was in very bad shape with lots of splices. This spring he purchased a Speedway EFI wiring harness plus an electronic controlled distributor and knock sensor & filter.
Normally I like to make only one change at a time, test it for proper operation, then make the next change. But in this case to keep the mechanical HEI distributor would need to wire up the 12 to 5 volt buffer circuit board into the new wiring harness between the existing distributor and the EBL. I decided to skip directly to adding the ESC distributor at the same time. Consequently I don't know if its a fuel problem or an issue with the new (used) ESC distributor. At first it seemed to run fine until the engine was hot, but now my understanding from Greg is it stumbles from cold.
The symptoms are somewhat like the issues I had when I installed my Howell system and Holley had reduced the size of the injectors from 80 down to 63pph. This Holley TB has the 80pph rounded top injectors. There is a fuel pressure gauge on the dash which shows it is running with normal fuel pressure. If it was a block fuel filter etc, I would expect to see the fuel pressure drop, but it stays steady.
So far we have:
- Installed a new ignition module (module from Dick Paterson)
- Rechecked the timing with a brand new timing light (6 degrees BTDC which matches the BIN)
- Disabled the knock sensor per RobR as it would randomly count a lot of knocks (Faulty module or sender?)
- swapped the coil/cap (originally we used the coil/cap from the existing HEI as we knew it was good)
- Checked voltages to coil, ECM/EBL and fuel pump.
- We have a new distributor pickup from DickP, but have not yet installed it.
- Rechecked all the connections
- Plus a number of other things checked and
rechecked
It appears I need to make the 4 hour drive to his place to help
troubleshoot the issue. Any ideas?
-- Bruce Hislop
James Hupy
Jun 2, 2024, 12:27:35 PMJun 2
to gmcm...@googlegroups.com
Bruce, be certain that the cooling system is completely full and under 9 psi pressure. If the temperature sensor for the howell system in not completely immersed in coolant, the 7747 goes "wonky" and cannot decide whether it should run in "cold start" mode or above 173° coolant temperature. The engine will run like you described. That issue tripped me up twice. Not gonna do it to me another time!
Might also be injectors that have dirty fuel.
Jim Hupy
Salem. Oregon
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Bill Van Vlack
Jun 2, 2024, 12:51:22 PMJun 2
to gmcm...@googlegroups.com
During my EFI installation, I ran the engine with carb and EBL spark control only. Depending on how hard it would be to temporarily fit a carb, that might be a way to isolate the issue.
Here's BobR's thoughts on EBL spark-only.
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