| [GMCnet] Fwd: [GMCMH-EFI] Gerald Downing EFI [message #325139] |
Wed, 18 October 2017 17:36 |
powerjon
Messages: 2446
Registered: January 2004
Karma: 5
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Senior Member |
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> Begin forwarded message:
>
> From: John Wright
> Subject: Re: [GMCMH-EFI] Gerald Downing EFI
> Date: October 18, 2017 at 9:20:45 AM EDT
> To: gmcmh-efi@googlegroups.com
> Cc: John Wright
>
> Ken,
> It looks an awful lot like the MPFI I have on our 455 except that I have a 4 barrel version. I would suggest that you contact Rance Baxter, the last email address that I had for him was rancefi@frontiernet.net
>
> Mine is a Accel Gen 6 based system. What year was the system installed on the engine?
>
> See answer below.
>
>
>
>> On Oct 18, 2017, at 8:35 AM, Ken Henderson wrote:
>>
>> Gerald Downing, from Dothan, AL, is visiting to see if we can get his '78 403's EFI working better. Until about February, he says the coach ran great, then began to miss erratically. He's had several people try to work on it, including Alex Ferrara, and an 84 yo former Olds mechanic from the '70's. No one has made any progress.
>>
>> The engine idles roughly, with a erratic miss -- NOT consistent as if coming from one cylinder. All of the spark plugs and wires, the ignition coil, cap, rotor, and the 7-pin module have been replaced. About the first thing I did after he opened the engine hatch was remove the distributor cap and shake the rotor. It moved side-to-side maybe 0.010"-0.015" -- too much, IMHO. I happened to have one, with no lateral play, so I installed it. If anything, the engine ran even worse!
>>
>> I then reset the timing as well as I could, by vacuum reading (his guage responds VERY slowly). Since I know NOTHING about the electronics of this system, I couldn't disable any ECU advance. Besides, there seems to be no timing pointer on the engine. Still no improvement. The engine stutters, stumbles, sometimes spits & backfires on low speed acceleration. When not under load, it runs perfectly -- smooth, quiet, with no indication of any problem. During acceleration, it begins to stutter again. At speed (50+ mph), it smooths out with only an occasional hiccup -- until a significant hill, then it stutters.
>>
>> We have found nothing to tell us where the EFI came from. It has a 2-barrel throttle body by Bendix and what appears to me to be a GM ECU, though I don't have a number from it. The photo below is not very good, but I'm hoping someone will recognize some part of the system.
>>
>> The aluminum fuel rails are connected at the front with a braided SS line. At the rear of each rail is another SS line with the two teed together; I PRESUME that's the return to the tanks. At near the center of the right rail is black cylinder, below the rail, with another SS line entering one end of the cylinder. The other end of the cylinder connects to a vacuum hose which, in turn, goes to the front of the throttle body; that cylinder is, I PRESUME, the manifold vacuum sensitive fuel pressure regulator; its location sort of baffles me though.
>>
> The Location of the fuel regulator is such that it controls the fuel pressure in the fuel rail by controlling the fuel return back to the tanks.
>
> I would suggest that you check the fuel pressure. It should be in the 42 to 55 psig range, anything lower than 40 psig will cause the engine to run ruffly and have no power. Ask me how I know. Mine turn out to be a fuel filter and bad HP fuel pump. If I am correct the fuel pump is a from a Mercedes 450 SEL. It should be a round silver pump with inlet and out let on either end.
>
>> I’m
>
>
>
> I have to run to my cardiac rehab exercise, so contact me later. I have information on most of the parts used on the fuel side of the system.
>
>
> JR
>
>
>
>> about at the end of my rope with the thing with NO documentation to help me and no knowledge of port injection, so any help will be appreciated.
>>
>
>> One thing we haven't tried, but plan to this morning, is replacement of the O2 sensor. I'll report the results of that.
>>
>> Oh yeah, one other thing I did: the round device just aft of the air inlet snorkel is the IAC, I finally figured out. Unlike our TBI systems, the IAC admits air through that little filter rather than using system air from the throttle body. That filter was VERY dirty, so I removed it and blew if out with compressed air. The pintle was completely black with carbon, which washed off with solvent. When first started and revved at idle, that seemed to correct the problem -- when driven, there was no improvement.
>>
>> Without help from Y'All, we have no idea what to do next except replace the system entirely. :-(
>>
>> Here's a shot of the system:
>>
>> https://drive.google.com/open?id=0B3XquSIYjDlecWFpbExxS1ZhbFk
>>
>> Ken H.
>> Americus, GA
>> '76 X-Birchaven w/Cad500/Howell EFI & EBL,
>> Manny Brakes & 1-Ton, Troy-Bilt APU, etc., etc., etc.
>> www.gmcwipersetc.com
>>
>> --
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>
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J.R. Wright
GMC GreatLaker
GMC Eastern States
GMCMI
78 30' Buskirk Stretch
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