Engine and Transmission Tech

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

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en
ian jenkins
Mississauga Ont canada
(67 posts)

Registered:
12/27/2010 11:06AM

Main British Car:
77 tr7

Low engine vacuum at idle
Posted by: en
Date: October 15, 2011 07:20PM

I've been working through this problem but could use some advice.
I have a 3.5 Rover with hot wire efi, 14CUX.
I don't know the history of this engine.
Approx 150 psi on compression test on all cylinders on cold engine. 40 psi fuel pressure at idle.
Idle speed keeps going up and down, i suspect ECU is trying to adjust settings. Ignition timing
is in the ball park
I have only around 5"vac at idle. Around 2000 rpm, i have 20"vac.
I get some backfire in plenum at idle, backfire out exhaust at higher rpm.
I blocked off all vacuum ports on plenum.

I've read that wrong cam timing can cause low vacuum at idle, but would vacuum be okay at higher rpm?

Any suggestions on what i should check adjust?
Thank yoian


tbush
Tim Bush
Minnesota, USA
(32 posts)

Registered:
11/16/2009 07:29PM

Main British Car:
'74 Spitfire, '59 Frogeye (sold) GM Vin T 3.1Lgoing in it.

authors avatar
Re: Low engine vacuum at idle
Posted by: tbush
Date: October 16, 2011 03:35PM

Ian, does the engine seem to run better and have more power when it is warm? If so I would check for a stuck or leaking EGR valve.


DiDueColpi
Fred Key
West coast - Canada
(1366 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Low engine vacuum at idle
Posted by: DiDueColpi
Date: October 17, 2011 12:38PM

Possibly something with your firing order or ign system?
Heavily retarded timing will also do this and so will "sadly" a flat camshaft.
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en
ian jenkins
Mississauga Ont canada
(67 posts)

Registered:
12/27/2010 11:06AM

Main British Car:
77 tr7

Re: Low engine vacuum at idle
Posted by: en
Date: October 17, 2011 02:34PM

I'm afraid i committed a common mistake.
I looked for something high tech when problem turned out to be a vacuum leak from the plenum heater.
Idling good now.


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