rough idle
Thread Starter
Intermediate
Joined: Apr 2003
Posts: 35
Likes: 0
From: Crestview FL
rough idle
Need some help diagnosing the problem. I have a stock 1985 auto that has a surging idle. Fuel pressure when vaccum is disconnected and plugged is 50 psi. Engine will surge between 500 and 800 rpms. If I remove the pcv valve from the drivers side valve cover the surge stops. If I plug the vacuum line connected to the pcv valve the engines stalls. Right now I am guessing that I have a leaky injector because when I turn the car off the fuel pressure drops to zero in about 15 seconds.
What do you guys think?
What do you guys think?
Drifting
Joined: Oct 2003
Posts: 1,594
Likes: 0
From: CALGARY ab
fuel pressure at the rail should stay up for awhile, it could be leaky injectors...
make sure that all the vaccums lines are connected and not leaking..this is the usual cause of idle surge...
it may be time to clean the TB, reset base idle IAC. check TPS setting.. at least this will eliminate one item that could be at fault.
make sure that all the vaccums lines are connected and not leaking..this is the usual cause of idle surge...
it may be time to clean the TB, reset base idle IAC. check TPS setting.. at least this will eliminate one item that could be at fault.
Pro
Joined: Mar 1999
Posts: 678
Likes: 0
From: Oslo, Norway
Hi.
I have an 85 4+3 and had some idle problems. Check the PCV valve by shutteling it and you should hear the ball inside rattle. Then it is most probably that the valve is ok. I had a fault in the EGR valve. Check it with applying vacuum at the valve. The vauum shall hold for about 20 sec. If not the valve is not working and has to be changed.
Then I saw that your fuelpressure is very high for stock to be. Remember that the injectors in an 85 is larger than all the later models and has a fuel pressure about 36PSI stock. The vauum line at the fuel pressure regulator shall decrease the pressure at idle. The pressure can be increased a bit to have a better spray pattern but 50 PSI is very high for a stock 85 thinking about the cam and heads. Some get idle problems with too high fuel pressure.
Then the vacuum lines. Check that there are no leakage and broken lines especially those under the plenum. They are brittle and breake very easy on that old car.. Then the easiest way to check for a bad idle. Rinse the throttle body and IAC valvefor carbon deposits. The engine is very sensitive for these carbon deposites. To check for false air around the plenum intake spray a little start gas around the runner/manifold to see if there should be any leakage. If leakage the engine will increase rpm when spraying startgas there.
There is, however , one thing you should check as a must. The TPS voltage should be ideal 0.54V or max 0,62 V. Do the minimum idle air adjustment as stated in the manual. A car as old as ours has most probably changed that as the throttlebody is worn and the idleair adjustment screw has been worn. the screw is situated on the right upper side of the throttlebody and if it has not been adjusted from a new car it hides behind a cover that can easyly be taken away with a screwdriver and plier.. It is a torx screw. The the min idle air adjsutment for an aoto is as far as I remember 400rpm
These are some points that can help you
jan-erik
I have an 85 4+3 and had some idle problems. Check the PCV valve by shutteling it and you should hear the ball inside rattle. Then it is most probably that the valve is ok. I had a fault in the EGR valve. Check it with applying vacuum at the valve. The vauum shall hold for about 20 sec. If not the valve is not working and has to be changed.
Then I saw that your fuelpressure is very high for stock to be. Remember that the injectors in an 85 is larger than all the later models and has a fuel pressure about 36PSI stock. The vauum line at the fuel pressure regulator shall decrease the pressure at idle. The pressure can be increased a bit to have a better spray pattern but 50 PSI is very high for a stock 85 thinking about the cam and heads. Some get idle problems with too high fuel pressure.
Then the vacuum lines. Check that there are no leakage and broken lines especially those under the plenum. They are brittle and breake very easy on that old car.. Then the easiest way to check for a bad idle. Rinse the throttle body and IAC valvefor carbon deposits. The engine is very sensitive for these carbon deposites. To check for false air around the plenum intake spray a little start gas around the runner/manifold to see if there should be any leakage. If leakage the engine will increase rpm when spraying startgas there.
There is, however , one thing you should check as a must. The TPS voltage should be ideal 0.54V or max 0,62 V. Do the minimum idle air adjustment as stated in the manual. A car as old as ours has most probably changed that as the throttlebody is worn and the idleair adjustment screw has been worn. the screw is situated on the right upper side of the throttlebody and if it has not been adjusted from a new car it hides behind a cover that can easyly be taken away with a screwdriver and plier.. It is a torx screw. The the min idle air adjsutment for an aoto is as far as I remember 400rpm
These are some points that can help you
jan-erik
