Auto transmission stutters while idling
Track Day
Joined: Mar 2011
Posts: 17
Likes: 0
Actually, reading all this thread again, i come across this post from Karl which I had missed yesterday :
"My '88 does this as well, but I think it is that the idle speed is too high: Enough so that the tranny is actually trying to move the car forward which I am preventing with the brakes. So the motor is pulling against the brakes. "
That feels like exactly what i'm having. If I had to intuitively tell whether my idle (in neutral) is too high or too low, I would probably say that it's a bit too high.
If Karl is seeing this... did you finally fix this issue?
That's great, I've got two hypothesis now, whereas yesterday at the same hour I was clueless with that problem .
"My '88 does this as well, but I think it is that the idle speed is too high: Enough so that the tranny is actually trying to move the car forward which I am preventing with the brakes. So the motor is pulling against the brakes. "
That feels like exactly what i'm having. If I had to intuitively tell whether my idle (in neutral) is too high or too low, I would probably say that it's a bit too high.
If Karl is seeing this... did you finally fix this issue?
That's great, I've got two hypothesis now, whereas yesterday at the same hour I was clueless with that problem .
Pro
Joined: Oct 2010
Posts: 678
Likes: 2
From: Indianapolis, In
I had this happen when I was first getting my 86.5 running. Sitting at a stop light it would surge against the brakes. I had 3 issues that where fixed.
1 High idle/ erattic idle- from vac leaks
2 Vac leak to trans.
3 low trans fluid.
Fixing 2 and 3 seemed to do the trick, and fixing 1 made everything better.
I rank them in that order because that was how I attacked the problem. 2 and 3 are easy to check if your not having any idle issues.
1 High idle/ erattic idle- from vac leaks
2 Vac leak to trans.
3 low trans fluid.
Fixing 2 and 3 seemed to do the trick, and fixing 1 made everything better.
I rank them in that order because that was how I attacked the problem. 2 and 3 are easy to check if your not having any idle issues.
Nordschleife Master
Joined: Jul 2011
Posts: 5,662
Likes: 26
From: North Country
Yves, ça va ?
I have an 87, but I think the vac line issue will be similar. The origin is off the manifold under the airbox from the 'T' multi-connector. It then runs over the bellhouse back to the transmission. It is not a single contiguous line. It goes to a metal hardline for a bit over the torque tube before going back to a hose-line when it reaches its destination at the vacuum modulator on the driver's side of the transmission. Therefore there are several places it can be compromised at these unions - it could also be compromised anywhere along the course of the hoses. I replaced the existing vac line with a single line like this - # 3 was the size - https://www.pegasusautoracing.com/pr...p?Product=3260
The vacuum modulator is the located on the driver's side of the transmission above the midway point of the ATF pan. In the pic attached it is green in color with a black rubber cap over the adjuster key - important that you must have an uncompromised rubber cap otherwise vacuum will be compromised.
With Mityvac, check vacuum at all segments to determine where 'leak' is. Also check to see if the vacuum modulator on the transmission holds vacuum - if cap is good and modulator does NOT hold vacuum you'll need a new one - they are available and not expensive.
Edit: With thinking about it, I may have been mistaken about the vac line over the torque tube - I know the ATF line over the tube reverts from hard to soft line - the vac line may very well be a single line IIRC - no real difference though when doing vacuum testing.
I have an 87, but I think the vac line issue will be similar. The origin is off the manifold under the airbox from the 'T' multi-connector. It then runs over the bellhouse back to the transmission. It is not a single contiguous line. It goes to a metal hardline for a bit over the torque tube before going back to a hose-line when it reaches its destination at the vacuum modulator on the driver's side of the transmission. Therefore there are several places it can be compromised at these unions - it could also be compromised anywhere along the course of the hoses. I replaced the existing vac line with a single line like this - # 3 was the size - https://www.pegasusautoracing.com/pr...p?Product=3260
The vacuum modulator is the located on the driver's side of the transmission above the midway point of the ATF pan. In the pic attached it is green in color with a black rubber cap over the adjuster key - important that you must have an uncompromised rubber cap otherwise vacuum will be compromised.
With Mityvac, check vacuum at all segments to determine where 'leak' is. Also check to see if the vacuum modulator on the transmission holds vacuum - if cap is good and modulator does NOT hold vacuum you'll need a new one - they are available and not expensive.
Edit: With thinking about it, I may have been mistaken about the vac line over the torque tube - I know the ATF line over the tube reverts from hard to soft line - the vac line may very well be a single line IIRC - no real difference though when doing vacuum testing.
Last edited by MainePorsche; Sep 18, 2014 at 10:18 AM.
