Engine Idle in Gear
Thread Starter
Intermediate
Joined: Dec 2009
Posts: 41
Likes: 0
Engine Idle in Gear
Hi everyone, I recently install a replacement GM 350 crate engine with 290 hp in my 79 Corvette with an automatic. Engine runs good and idles at 900 rpm with 15 inches of vacuum. When I put it in gear the engine drops to 700 rpm and barely runs. I called the GM tech guy and he said the stock stall converter is too low. He suggested 2,500 rpm converter. Has anyone else had this experience with this particular engine? This doesn't sound right since the cam is identical to the L-82 specs and many Corvettes had automatics. Any comments would be greatly appreciated. Thanks in advance.
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
I have a 79, L-82 with a 350 automatic transmission. The manual calls for 700 RPM in gear. My car does well at 700RPM at idle. If you raise the idle too much with a stock converter you have to stand on the brake at red lights.
Thread Starter
Intermediate
Joined: Dec 2009
Posts: 41
Likes: 0
Thanks for the info. You have confirmed my thinking that it should idle at 700 rpm in gear with the stock converter. I will have to keep digging to find out why it wants to stall. Carburetor was recently rebuilt and the engine idles smoothly at 900 rpm out of gear. Can't find any vacuum leaks, but will keep checking.
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Converter could be causing idling problems if I remember correctly. (I'm not an auto guy) But only if it's worn out, not because of the stall speed.
If it worked before the new motor, the problem is in the tune. What do you mean barely runs? Runs rough?
If it worked before the new motor, the problem is in the tune. What do you mean barely runs? Runs rough?
Thread Starter
Intermediate
Joined: Dec 2009
Posts: 41
Likes: 0
When I put it in gear, the engine idle goes down to around 500 rpm and it won't idle that low, so it ties to idle but dies out. Starts right back up. It seems to idle just fine in gear at 1,000 rpm, but then the out-of-gear idle is about 1,300. I am going by the in car tach and I am starting to question whether it is reading right. I already know the speedometer is off by at least 10 mph as it reads 70 mph at 2,500 rpm. I am going to try my engine analyzer tach to see what it reads. Probably should have done that long ago.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
If the transmission has some age [and/or hard use] on it, some of the clutch plates may be burnt and warped. If so, when you bring on the "drive" clutch, another clutch may be dragging. Two clutches need to be on at one time to transfer any power through the tranny. If another clutch is partially "on" (dragging) you will end up with the problem you are experiencing. Check out the vacuum, carb adjustment, and tune-up possibilities first. If you exhaust those possibilities, it may be a tranny problem.
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Drifting
Joined: Jan 2002
Posts: 1,645
Likes: 11
From: Schaumburg IL
Like everyone said - timing, curb idle speed, plugs and wires etc.
Put a vacuum gauge on the intake and see what the vacuum is at idle and in gear at idle. Two things come to mind - idle will drop in gear and if you have a vacuum leak inside the power brake booster it will suck off the vacuum and cause trouble. Listen closely inside the car and see if you hear a hiss of air. Does the brake peddle stay solid or tend to bleed off?
Another area in gear is the vacuum modulator on the trans can cause a leak and drop the vacuum.
Put a vacuum gauge on the intake and see what the vacuum is at idle and in gear at idle. Two things come to mind - idle will drop in gear and if you have a vacuum leak inside the power brake booster it will suck off the vacuum and cause trouble. Listen closely inside the car and see if you hear a hiss of air. Does the brake peddle stay solid or tend to bleed off?
Another area in gear is the vacuum modulator on the trans can cause a leak and drop the vacuum.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Hi everyone, I recently install a replacement GM 350 crate engine with 290 hp in my 79 Corvette with an automatic. Engine runs good and idles at 900 rpm with 15 inches of vacuum. When I put it in gear the engine drops to 700 rpm and barely runs. I called the GM tech guy and he said the stock stall converter is too low. He suggested 2,500 rpm converter. Has anyone else had this experience with this particular engine? This doesn't sound right since the cam is identical to the L-82 specs and many Corvettes had automatics. Any comments would be greatly appreciated. Thanks in advance.
