help me with a vacum problem?

[copedrake]

hey i recently had a new distributer put in by a mech. and when i got my 85 sierra back (350) it ran with the new distributer but the check engine light came on when accelarating and when doing this it sometimes backfires and stuters. but when up to speed it runs fine.

i bought a vacum gauge ( damn there more then i thought) and hooked it up to the pcv. it came out to be 3" at idel and and would go anywhere from 3 to 5 when giving it gas. i pulled out the vacum diagram and checked that all the hoses where hooked up corectly and not broken. according to what i have read about vacum gauges the reading i am getting says to check all hose hook ups and that there not broken.

any ideas? ive told the mech. that put my new distributer in what has happened and he told me to do what i have already done.

BTW- there is a small but steady fuel leak from the gas line going directly into the carb, not sure if that would be directly related to my symtoms.

 

[sled_dog]

how does it run at idle? 3" is HORRIBLE at idle. I'd barely expect it to be running.

 

[copedrake]

yea it runs horribly at idel right when it is started up, if i run it for a while on the road it evens out, but i have yet to check the vacum after running it after awhile.ill be back with the reading.

 

[copedrake]

i just took it for a run and tested it when i got back. it read the same, however if i pulled the insert in and out from the pcv the got up to 8 and 10. but at idel it was the same.

 

[diesel4me]

low vacuum not good..

If your getting less than 15-17" of vacuum at idle,something is definately wrong..I'd use a different source of manifold vaccum other than the PVC valve hose for test purposes,and leave the PVC valve intact and hooked up..

I'd start by spraying carb cleaner on the carb base gasket,intake manifold gaskets,and all the fittings that screw onto the carb and intake,with engine running at idle.--also any item with a vacuum hose on it..or use a stethascope to listen for leaks--a peice of 1/4"hose stuck in your ear works well in a pinch--but be careful ,it can conduct the HEI voltage to your eardrums!

--if they all check ok (no change in idle speed )then other causes of low vacuum are poor compression,late ignition timing,and a cracked intake under the valley where the heat riser passages are..also check the EGR valve (if you still have one!) and ensure its closing all the way at idle..clogged catalitic converter or squashed exhaust pipes can cause low vacuum readings too..

 

[copedrake]

i have the egr valve still, what should i look for for it to be good or bad? also, how do i check the compression or the timing. the mech. put the the didtrib. in so i assume that the timeing is alright. thanks alot for the help!

 

[diesel4me]

how 2

To check the EGR for closing fully,warm the engine to normal operating temparature,and use gloves or a rag to protect your fingers from burns..by placing your fingers under the EGR valve and lifting up on its diaphram (accessable by the openings underneath it) the motor should stumble and stall if it was closed as it should have been..if not,its probably clogged up with carbon underneath where its bolted on,or its failed in the "open" position..

I'd guess the gauge is hooked up to the wrong vacuum source..most engines wouldn't start or idle with only 3" of vacuum..it would have to be a HUGE vacuum leak that would be very apparent--you'd hear suction !!..