MG Sports Cars

engine swaps and other performance upgrades, plus "factory" and Costello V8s

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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

What causes a dip in idle speed when coming to a halt with the clutch in
Posted by: limey222
Date: October 17, 2015 08:10PM

My 94 Camaro 3.4 conversion is working very well. The only minor cause for concern is that when I pull up to a stop sign or traffic signal and put the clutch in, the idle dips maybe 50 -75 rpm below its normal level very briefly and corrects itself. It always causes me to panic that the engine might cut out but it never does. The normal idle appears to be around 600 - 650 rpm. I suspect my tach might be adding little slow right now and needs fine tuning. I put the car through the 10 min/5 mins IAC reset procedure ( I think thats what its called) and it seemed to learn the idle setting OK a couple of weeks ago.


Scott68B
Scott Costanzo
Columbus, Ohio
(527 posts)

Registered:
10/25/2007 11:30AM

Main British Car:
1968 MGB GM 5.3 LS4 V8

authors avatar
Re: What causes a dip in idle speed when coming to a halt with the clutch in
Posted by: Scott68B
Date: October 17, 2015 08:34PM

Are you running a vehicle speed sensor and is the PCM "aware" that it is there? No VSS signal could cause the symptoms you're describing.


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: What causes a dip in idle speed when coming to a halt with the clutch in
Posted by: limey222
Date: October 18, 2015 12:34PM

No I'm not. I was advised by Brian at BMC Autos that it wasn't necessary. Is there a sensor on the 95 WC T-5? I can't remember if his aftermarket wiring harness has a dedicated wire from PCM, I'll check. How easy is it to hook up and what are the pitfalls or potential problems I might encounter. My car is OBD1.5 so it's not easy to trouble-shoot if things go wrong. Since it is running quite well right now I'm nervous to risk complicating things.


Scott68B
Scott Costanzo
Columbus, Ohio
(527 posts)

Registered:
10/25/2007 11:30AM

Main British Car:
1968 MGB GM 5.3 LS4 V8

authors avatar
Re: What causes a dip in idle speed when coming to a halt with the clutch in
Posted by: Scott68B
Date: October 18, 2015 09:47PM

Quote:

No I'm not. I was advised by Brian at BMC Autos that it wasn't necessary. Is there a sensor on the 95 WC T-5? I can't remember if his aftermarket wiring harness has a dedicated wire from PCM, I'll check. How easy is it to hook up and what are the pitfalls or potential problems I might encounter. My car is OBD1.5 so it's not easy to trouble-shoot if things go wrong. Since it is running quite well right now I'm nervous to risk complicating things.

Michael,

It might be issues other than a missing VSS. All I can tell you is my V6 exhibited the same symptoms and adding a VSS solved the issue and made for a much better running engine....YMMV. If I were you I'd work with Brian, he's probably seen this before. You could always live with it too.

It isn't terribly difficult to add a VSS, your T5 came with one, is it still there? If so, you would need to wire it to the proper pins in the PCM. Unless your rear end gearing and tire sizes are the same as the donor vehicle, it isn't likely that the PCM will see an accurate vehicle speed but that might not matter. It might be enough for it to see the vehicle is moving. That would be a risk though but you could gat a tuner to adjust the calibration in the PCM to correct it.

What makes OBD1.5 so difficult to troubleshoot?


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: What causes a dip in idle speed when coming to a halt with the clutch in
Posted by: limey222
Date: October 19, 2015 01:01PM

Scott, I sent you a PM


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