85 5.8L issue
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At idle, although seems to be intermittent at speed. Came on all of a sudden as it has run fine for years. Now also loads up on fuel and must not touch accelerator when starting. Starts rough then clears out but yet surges. Rudimentary vacuum leak tests show no leaks. Suspect something in the emissions / EGR system. Would like to remove all of the miles of vacuum fittings, solenoids etc and get back to the basics
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At idle, although seems to be intermittent at speed. Came on all of a sudden as it has run fine for years. Now also loads up on fuel and must not touch accelerator when starting. Starts rough then clears out but yet surges. Rudimentary vacuum leak tests show no leaks. Suspect something in the emissions / EGR system. Would like to remove all of the miles of vacuum fittings, solenoids etc and get back to the basics
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Have you tried disconnecting and plugging the vacuum line to the EGR to see if it smooths out? Are you using a vacuum gauge to test vacuum, or disconnecting lines and seeing if there's a reaction?
EDIT:
Missed this before, this makes it sound like it's more likely a carb issue.
EDIT:
Missed this before, this makes it sound like it's more likely a carb issue.
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Not yet but will try tomorrow.. Using ether spray at fittings to see if engine speeds up. Quick check without vacuum gauge. Another suggestion was carb power valve. Will also check that out. Also have read where the Map sensor causes similar conditions but cannot locate. Wonder if one is used on this system?
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Shouldn't have any sensors to worry about. Was working on my cousin's 85 with the same 351w HO earlier today.
Power valve, floats sticking, etc can cause all kinds of issues. If it was ever rebuilt in the past, the plugs that cover the mixture screws will have been removed. If you turn them (or at least the driver's side one) all the way in it should kill the motor if the power valve is functioning. If it keeps stumbling along and trying to run, I'd recommend picking up a rebuild kit for the carb.
Power valve, floats sticking, etc can cause all kinds of issues. If it was ever rebuilt in the past, the plugs that cover the mixture screws will have been removed. If you turn them (or at least the driver's side one) all the way in it should kill the motor if the power valve is functioning. If it keeps stumbling along and trying to run, I'd recommend picking up a rebuild kit for the carb.
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An EGR valve that's stuck open will cause hard starting and a rough idle.
The EGR valve operates only when the engine is warm and at speed, never at idle (unless it's carboned up and stuck open).
You can look at the vacuum diaphragm and see what's happening... with the engine warm, open the throttle to about 2,000 RPM with your hand while you watch that diaphragm, watch it open & close the valve.
Can do the same thing manually if you have a vacuum pump.
I personally think the carb needs more attention than any of the emissions controls.
The EGR valve operates only when the engine is warm and at speed, never at idle (unless it's carboned up and stuck open).
You can look at the vacuum diaphragm and see what's happening... with the engine warm, open the throttle to about 2,000 RPM with your hand while you watch that diaphragm, watch it open & close the valve.
Can do the same thing manually if you have a vacuum pump.
I personally think the carb needs more attention than any of the emissions controls.
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From: Oregon
Get a real Holley kit, the cheap-o rebuild kit my cousin got for his had a garbage power valve (not even any numbers stamped on it) in it that wouldn't let it run right. Wasted a LOT of time tuning before I pulled a single stage valve out of a Holley I had on the shelf and it now runs like a dream.
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