Erratic Idle
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7th Gear
Joined: Aug 2002
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Erratic Idle
Lately, my 1980, 5-speed has been having an idle speed problem. Even after a reasonable warm-up period, my idle is all over the place. It will idle at 1300 rpm; then drop to 700 rpm, only to find a different idle speed the next time I come off the pedal. Generally, after decelerating from highway speed to a stop, the rpm will remain at around 1500 rpm for a few minutes before dropping to around 900. Need to have it smogged this week, and I'm not sure I can get the idle speed to remain stable enough to pass the test! Help!
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From: The Vacation State
Couple of suggestions that you can do without really tearing into it.
Any vacuum leaks?
Is your engine properly timed? Check spark advance as that has a really big impact on idle speed. It needs to be timed revved up and then checked at idle.
The Manual 24-1 reads, 750 +/- 50 rpm at idle, from mod 1983 650 +/- 50 rpm. The Manual 28-23 reads, 23 degrees before TDC at 3000 rpm with both vacuum hoses removed. Then it should settle around to 10 at idle speed.
Next would be to to check to see to if fuel is properly getting to the injectors. I posted this part this week
1. Remove the cap from the end of the fuel rail. Don't loose the sealing ball.
2. Attach your gauge to the end of the fuel rail.
3. Pull up foot support in footwell on passenger's side and pull fuel pump relay off the Board
4. Bridge Terminals 30 and 87 with extral wire. Fuel pump should run.
5. Check values fuel gauge readings against - engine stopped, relay jumpered values in the manual.
6. Remove Jumper, install relay, start engine
7. Check values fuel gauge reading against - engine running at idle value in the manual.
Values which are significantly higher than then specs in manual indicate a problem with the fuel pressure reg.
Values which are significantly lower then specs in maual indiate a problem with fuel pump or clogged filter.
8. After engine has been run at idle, stop engine and allow the car to sit for 20 minutes to check the leakdown.
9. Check values fuel gauge reading against - 20 minute leakdown value in the manual.
Values significantly below specs in manual are the result of a bad check valve.
Rez
Any vacuum leaks?
Is your engine properly timed? Check spark advance as that has a really big impact on idle speed. It needs to be timed revved up and then checked at idle.
The Manual 24-1 reads, 750 +/- 50 rpm at idle, from mod 1983 650 +/- 50 rpm. The Manual 28-23 reads, 23 degrees before TDC at 3000 rpm with both vacuum hoses removed. Then it should settle around to 10 at idle speed.
Next would be to to check to see to if fuel is properly getting to the injectors. I posted this part this week
1. Remove the cap from the end of the fuel rail. Don't loose the sealing ball.
2. Attach your gauge to the end of the fuel rail.
3. Pull up foot support in footwell on passenger's side and pull fuel pump relay off the Board
4. Bridge Terminals 30 and 87 with extral wire. Fuel pump should run.
5. Check values fuel gauge readings against - engine stopped, relay jumpered values in the manual.
6. Remove Jumper, install relay, start engine
7. Check values fuel gauge reading against - engine running at idle value in the manual.
Values which are significantly higher than then specs in manual indicate a problem with the fuel pressure reg.
Values which are significantly lower then specs in maual indiate a problem with fuel pump or clogged filter.
8. After engine has been run at idle, stop engine and allow the car to sit for 20 minutes to check the leakdown.
9. Check values fuel gauge reading against - 20 minute leakdown value in the manual.
Values significantly below specs in manual are the result of a bad check valve.
Rez
Pro
Joined: May 2003
Posts: 601
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From: SF Bay Area
Henry,
I have been working through a number of issues with the 1980 that sound similar. I am not finished, but here is what I have done and found so far.
1) Vacuum leak check
I first checked all the hoses and found one that was attached to the throttle body that used to have a clamp on it. I dredged the clomp out of the oil glop on the top of the engine and re-installed it. Next I followed the procedure in Nichols to check the throttle body for leaks. This basically covering the input and applying pressure. I found that the O ring at the base of the AFM was not sealed well and have a new one on order.
2) I checked the cold start sensor which worked fine, and the cold start injector. No problems found
3) Checked Temperature Sensor number 2 which was also working as advertised.
4) Checked the Decel valve. (Pass side under spider, looks like a UFO)
You do this by pulling the vacuum tube while you are idling and pumping it up with a might vac (get one if you don't have one). As I did this, BINGO, the idle started to oscillate on me from about 200 to 1800 rpm about once per second. At the sage advice on one of the rennlisters, I pulled the thing out and plugged the vacuum line and the two hoses that attach to the throttle body. I believe this has fixed my surging idle. BTW, this part was supposedly removed from later models and doesn't seem to effect performance. Its only job is to smooth out the rpm when you decelerate.
I still have an idle which is a bit high and will settle anywhere from around 800 to 1200 rpm depending on its mood. I am hoping the O ring will fix it.
Next steps for me are to replace the O ring when it arrives, then check the timing, and adjust the idle and mixture.
HTH
I have been working through a number of issues with the 1980 that sound similar. I am not finished, but here is what I have done and found so far.
1) Vacuum leak check
I first checked all the hoses and found one that was attached to the throttle body that used to have a clamp on it. I dredged the clomp out of the oil glop on the top of the engine and re-installed it. Next I followed the procedure in Nichols to check the throttle body for leaks. This basically covering the input and applying pressure. I found that the O ring at the base of the AFM was not sealed well and have a new one on order.
2) I checked the cold start sensor which worked fine, and the cold start injector. No problems found
3) Checked Temperature Sensor number 2 which was also working as advertised.
4) Checked the Decel valve. (Pass side under spider, looks like a UFO)
You do this by pulling the vacuum tube while you are idling and pumping it up with a might vac (get one if you don't have one). As I did this, BINGO, the idle started to oscillate on me from about 200 to 1800 rpm about once per second. At the sage advice on one of the rennlisters, I pulled the thing out and plugged the vacuum line and the two hoses that attach to the throttle body. I believe this has fixed my surging idle. BTW, this part was supposedly removed from later models and doesn't seem to effect performance. Its only job is to smooth out the rpm when you decelerate.
I still have an idle which is a bit high and will settle anywhere from around 800 to 1200 rpm depending on its mood. I am hoping the O ring will fix it.
Next steps for me are to replace the O ring when it arrives, then check the timing, and adjust the idle and mixture.
HTH
Pro
Joined: May 2003
Posts: 601
Likes: 0
From: SF Bay Area
One other thing you should check is the idle switch. Mine was toast. If you look at where the accelerator cable comes into the throttle body you will see two switches. The top one is for wide open throttle and should click in around 70% of throttle or so. The bottom one is for the idle. It should click right at you approach no throttle.
With the car off, when you move the cable by hand you can hear these click. Check to make sure they are working and not loose.
With the car off, when you move the cable by hand you can hear these click. Check to make sure they are working and not loose.
