Engine and Transmission Tech

tips, technology, tools and techniques related to vehicle driveline components

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limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

"Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: limey222
Date: September 27, 2015 12:26PM

First let say that I have really appreciated all of the help I have received from site members during this project.
It was a sunny day here n Portland yesterday and in celebration of solving my front disc brake lock-up problem I actually got to take it for a 4 mile test drive.
It felt pretty darn good even took it up to 75 for short time and reached 5th. Things were really boding well for all my effort so turn home and about block from my house pulling away from a stop sign the engine suddenly cut out. I coasted to the curb and tried a restart, it caught after a few seconds but all was not well th idle was really rough, almost like it was firing on say 4 cylinders only. I tried to pull away and the revs dropped to about 250 and almost stalled. I gave it a lot more gas and eventually managed to get it rolling but it was struggle to keep it going all the way home. It was misfiring and threatening to stall all the way.
When I got home I checked all of the obvious things but could find nothing out of place. I have installed a check engine light wired from BMCAutos harness but I haven't been sure if it was working or not. I have used an small LED light and wasn't sure if should have been convention bulb. Anyway the light never came on.

The reality suddenly hit me that this was what I was most concerned about when first contemplating keeping the stock FI system or changing over to distributor/carb setup. Without have an OBD-1 port I have no way to get any pointers where the problem(s) lies. I went online last night and ordered a OBD-1 and OBD-2 combination scan tool and resolved to install the OBD-2 socket that I had previously purchased today to the wiring instructions that Brian had given me. I also downloaded and printed off a 4 page list of OBD-1 fault explanations.

In the meantime if anyone can give me some pointers of things I can check toady I would be most appreciative. Most of the sensors are new ones installed when I put the engine in. The gas feed is all new from the new 18 gal tank, hi-pressure pump and filter, new sell lines right up the the fuel rail. The coil pack, plug cables and plugs are all new. Th engine had been running very smoothly up to this point. When I was restarting the car back in my driveway my wife who was behind the car said that there was a lot of strong fumes which I take to mean probably a rich mixture. Has the PCM for some reason switched to a default mode of operation. If so can it be reset etc etc.
FullSizeRender 7.jpg



Edited 3 time(s). Last edit at 09/27/2015 05:54PM by limey222.


Nexxussian
Erik Johnson
Alaska
(62 posts)

Registered:
04/20/2015 10:32PM

Main British Car:
1974, MGB, Citroen Color Rover V8

Re: Finally took my new 94 GM 3.4 conversion MGB out for a spin..all good then big problem
Posted by: Nexxussian
Date: September 27, 2015 04:17PM

I mean no disrespect, but have you checked how much gas is in the tank?

Pumping air and fuel could cause an issue like that for a very short period of time (less than a block, right?).

I ask as it wouldn't be the first "extended" test drive I've heard of to run out just short of home.

That, and it's the simplest, easiest and cheapest issue to fix.

I prefer to check those items first. ;)

Best of luck, I hope it turns out to be simple. :)


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Finally took my new 94 GM 3.4 conversion MGB out for a spin..all good then big problem
Posted by: limey222
Date: September 27, 2015 05:36PM

I adjusted the the new fuel sender to show empty at just about the 2-1/2 gallon mark before I installed the new 18 gal aluminum tank and I also added an additional 1-1/2 gals from a gas can just before the drive so even though I would like it to be something that simple I know it isn't.



Edited 1 time(s). Last edit at 09/27/2015 07:12PM by limey222.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6468 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: "Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: BlownMGB-V8
Date: September 28, 2015 10:46AM

Check fuel pressure?


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: "Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: limey222
Date: September 28, 2015 03:04PM

Jim,

Purchased a fuel pressure gage and hooked it up.
With just ignition turned on (pump actuated for about 2 secs)....40 psi
Started engine (cold) at idle 41 psi.
increased revs to 2000...41 psi

Do I need to take measurements when engine is hot as well?


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: "Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: limey222
Date: September 28, 2015 07:07PM

Dug in with some more trouble-shooting this afternoon. Now that it appears that the fuel pressure is OK so I decided to pull the spam plugs. To my surprise plugs 1 & 4 were very sooty whereas al the other plugs were clean, wet and smelled of gas. Now plugs 1 & 4 are connected to the lowest of the coils in th pack so I naturally assumed that that coil, which along with the other two were all new about 6 months ago. So I replace it with a new one totally expecting to have the problem solved...but no, the car still ran very erratically, stumbling and threatening to stall all of the time. So my question is what else could cause just those two plugs to soot up, they are in different banks. Your suggestions will be really welcome at this point because I'm at loss.
Plugs 1 & 4.JPG
Plugs 2-3-5-6.JPG



Edited 1 time(s). Last edit at 09/28/2015 07:09PM by limey222.


Nexxussian
Erik Johnson
Alaska
(62 posts)

Registered:
04/20/2015 10:32PM

Main British Car:
1974, MGB, Citroen Color Rover V8

Re: "Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: Nexxussian
Date: September 28, 2015 07:58PM

Am I reading that correctly?

You have two plugs that are sooty, indicating they were firing, albeit too rich.

The other 4 plugs are "white" (the picture looks like they are fresh out of the box) and smell like gasoline?

Sounds like you have 4 cylinders flooded (no spark = no burn = no soot).

If that is the case, try cleaning the two sooty ones, drying the 4 white ones, clearing any fuel from the cylinders (perhaps motor it on the starter with the plugs out and coils disconnected) reassemble and try it again.

I can't say why they flooded, but 4 of 6 holes pumping unburned air out the exhaust will read as extreme lean to the computer

At that point, I would expect that system to enrich as far as it can, giving the symptoms you describe.

If this makes it better for now, you will want to troubleshoot until you find what caused the 4 to flood in the first place.

Best of luck with it.



Edited 1 time(s). Last edit at 09/29/2015 12:56AM by Nexxussian.



limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: "Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: limey222
Date: September 29, 2015 04:13PM

Looks like I found the problem. It was the new coil module (the base that the coils are bolted and connected to. I just had a hunch and since I still had the used one that came with the engine I swapped them out. An immediate difference, like night and day, the idle settled down to a very smooth rhythm, the lumpiness was improved by about 90%. Can I improve things more...probably, but I just so happy to have solved the problem.
I them put the engine through the 10 and 5 minute routine to allow it to reprogram itself. During that time the idle speed varied up and down and the mixture seem to change as there were short periods when the exhaust smoked and became clean again.
Next step in a short while is to road test it. If all is good then I can tied up the under dash wiring again since I snipped all of the tie-wraps when I was trouble shooting. The PCM is mounted under the dash on the passenger side along with the fuse and relay panel.
IMG_0844.JPG



Edited 2 time(s). Last edit at 09/29/2015 04:22PM by limey222.


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: "Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: limey222
Date: September 29, 2015 06:08PM

Just aheads up to everyone that helped out.

Just took my car around to the shop that a friend runs that specializes in British vintage cars now that I got the bugs worked out. he helped with the heavy stuff like pulling the original engine and transmission and installing the GM 3.4 and T5. I handed him the keys and said "now take it for a test drive, needless to say I went with him. He was hooting like a little kid with a big smile on his face, especially when he floored it turning into a side street and powered it sideways. He turned to me and said I can't do that in my MGB's.
Took the car back home and gave it a well deserved wash and polish after months of gathering grime in the garage while doing the conversion. It's a beautiful sunny day here in Portland so I plan on surprising my wife and taking her for a riverside drive tonight. Tidying up the wire that still hanging down under the passenger side dash can wait until tomorrow night.


rficalora
Rob Ficalora
Willis, TX
(2764 posts)

Registered:
10/24/2007 02:46PM

Main British Car:
'76 MGB w/CB front, Sebring rear, early metal dash Ford 302

authors avatar
Re: "Requesting advice on 3.4L diagnostics, with or without OBD."
Posted by: rficalora
Date: September 29, 2015 11:40PM

Congratulations Michael! Feels great when you solve a problem, doesn't it!


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