Idle issue - drops rpm but recovers

yale · 05-21-2016, 12:56 PM

So my '87 runs great down the road, no issues, nice and strong. However sitting still, in gear (auto trans) she will idle and suddenly the rpms will drop, but she'll "catch it" and recover. She'll do that once or twice a traffic light, never quite dying each time. If i put her in neutral the drop in rpm isn't as bad.
I have checked vacuum lines, air flow, and the like but found nothing. Thinking it might be the fuel pressure regulator. I have never replaced it or or the fuel filter (but have only had her for 7 months).

Thoughts?

yale · 05-24-2016, 10:01 PM

Update: Just finished a two day job of replacing the fuel filter. Not sure if it was by design but when I loosed the lines (which wasn't too bad) the fuel seemed to be on a constant siphon so I had fuel running constantly down my arm. In that confusion I managed to drop the little copper washer and got to wait a day until I could replace it.
Anyway, replacing the fuel filter didn't make any difference. I now have a new fuel pressure regulator so that is next. Not this weekend though as I'm taking her up to Limerock Park for the Pirelli World Challenge races.

Mybestsupra2 · 05-27-2016, 08:23 PM

Hi, there are many items that need to be checked for a situation like yours. My feeling would be that the fuel reg is not the problem, as it runs fine down the road. But you could check the fuel pressure following the TSRM before wasting money on parts.
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/
If you are sure that there are no vac leaks, and the timing and advance are spot on, the next best place to check is the AFM (if a Turbo it's a Karman Vortec System, and do not try to clean it with anything). Check it using the TSRM specs. The next thing to check is the TPS. Using the TSRM specs make sure it is in spec. And then the IAC, mostly that get very dirty and a good cleaning will get it working right again. My best guess is that your problem will be in one of these areas. One other item that sometimes makes a difference is the Dashpot. It allows the idle to come down slowly when coming off the throttle.
Luck

yale · 06-11-2016, 12:44 AM

I checked the resistance in the AFM and all values were way below the suggested values. Thinking that may be the issue now.
I did replace the fuel pressure regulator, just because I had a new one on hand. However I haven't been able to test it as the front end is up in the air while I rebuild it.

Mybestsupra2 · 06-13-2016, 04:20 PM

Hi, it may be that the afm is bad, but before I would spend a lot of money trying to find a replacement, I would
1, make very sure that all the clamps on the air hoses and turbo hoses,(if a turbo), are tight. Makes a big difference.
2, Check and adjust if necessary the TPS using the TSRM as a guide. The 87 Service Manual is not correct in the TPS adjusting diagram.
3, Remove and clean the IAC, being careful to not lose the grommet and filter if it is a turbo.
The AFM's don't go bad that often, mostly what happens is guys try to clean them, and ruin them.
The AFM on the 7mge,(non turbo), can be gently cleaned with the proper solution, gently and not touching the wires with anything but a gentle spray and gentle air to dry.
The AFM on the 7mgte Turbo will most likely be ruined by attempts at cleaning. Many have tried and failed to clean this one. Best not to try.
I believe cleaning the iac, and adjusting the tps and tightening all hoses will fix your problem.
Luck to all.

Bru · 07-09-2016, 03:16 AM

Another thing to take a peek at is the function of the EGR valve if you still have one.
Run a vacuum line separately down to the vacuum actuator on the valve and put a mouth vacuum on it until it retracts completely to check that there are no air leaks in the diaphragm. Quickly release the vacuum to allow the spring to bust any carbon that might have accumulated on the valve seat inside. Do that a couple times with the engine not running. The manual test for the EGR valve function is to start the engine and then apply a vacuum to the diaphragm at idle. The engine should stall. To test the vacuum modulator follow the test procedure in the book but a real quick check is to apply mouth pressure to the connection on the bottom. If there's a leak in the diaphragm inside the modulator, air will pass through. Otherwise, it should hold pressure if it's working properly. I had a problem with a hole in this diaphragm which caused the engine to run funny at speed with no load on. It felt like the engine was pulling and pushing. When I replaced the defective vacuum modulator, the problem went away.

05-21-2016, 12:56 PM	#1
yale Intake Join Date: Oct 2015 Location: CT Posts: 38	Idle issue - drops rpm but recovers So my '87 runs great down the road, no issues, nice and strong. However sitting still, in gear (auto trans) she will idle and suddenly the rpms will drop, but she'll "catch it" and recover. She'll do that once or twice a traffic light, never quite dying each time. If i put her in neutral the drop in rpm isn't as bad. I have checked vacuum lines, air flow, and the like but found nothing. Thinking it might be the fuel pressure regulator. I have never replaced it or or the fuel filter (but have only had her for 7 months). Thoughts?

05-27-2016, 08:23 PM	#3
Mybestsupra2 Stock Join Date: Feb 2016 Location: Pa Posts: 2	BAd Idle Hi, there are many items that need to be checked for a situation like yours. My feeling would be that the fuel reg is not the problem, as it runs fine down the road. But you could check the fuel pressure following the TSRM before wasting money on parts. http://www.cygnusx1.net/Supra/Library/TSRM/MK3/ If you are sure that there are no vac leaks, and the timing and advance are spot on, the next best place to check is the AFM (if a Turbo it's a Karman Vortec System, and do not try to clean it with anything). Check it using the TSRM specs. The next thing to check is the TPS. Using the TSRM specs make sure it is in spec. And then the IAC, mostly that get very dirty and a good cleaning will get it working right again. My best guess is that your problem will be in one of these areas. One other item that sometimes makes a difference is the Dashpot. It allows the idle to come down slowly when coming off the throttle. Luck

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05-24-2016, 10:01 PM	#2
yale Intake Join Date: Oct 2015 Location: CT Posts: 38	Update: Just finished a two day job of replacing the fuel filter. Not sure if it was by design but when I loosed the lines (which wasn't too bad) the fuel seemed to be on a constant siphon so I had fuel running constantly down my arm. In that confusion I managed to drop the little copper washer and got to wait a day until I could replace it. Anyway, replacing the fuel filter didn't make any difference. I now have a new fuel pressure regulator so that is next. Not this weekend though as I'm taking her up to Limerock Park for the Pirelli World Challenge races.

06-11-2016, 12:44 AM	#4
yale Intake Join Date: Oct 2015 Location: CT Posts: 38	I checked the resistance in the AFM and all values were way below the suggested values. Thinking that may be the issue now. I did replace the fuel pressure regulator, just because I had a new one on hand. However I haven't been able to test it as the front end is up in the air while I rebuild it.

06-13-2016, 04:20 PM	#5
Mybestsupra2 Stock Join Date: Feb 2016 Location: Pa Posts: 2	Hi, it may be that the afm is bad, but before I would spend a lot of money trying to find a replacement, I would 1, make very sure that all the clamps on the air hoses and turbo hoses,(if a turbo), are tight. Makes a big difference. 2, Check and adjust if necessary the TPS using the TSRM as a guide. The 87 Service Manual is not correct in the TPS adjusting diagram. 3, Remove and clean the IAC, being careful to not lose the grommet and filter if it is a turbo. The AFM's don't go bad that often, mostly what happens is guys try to clean them, and ruin them. The AFM on the 7mge,(non turbo), can be gently cleaned with the proper solution, gently and not touching the wires with anything but a gentle spray and gentle air to dry. The AFM on the 7mgte Turbo will most likely be ruined by attempts at cleaning. Many have tried and failed to clean this one. Best not to try. I believe cleaning the iac, and adjusting the tps and tightening all hoses will fix your problem. Luck to all.

07-09-2016, 03:16 AM	#6
Bru Bone Stock w Upgrades ;-) Join Date: Apr 2010 Location: Tampa Bay Posts: 210	Another thing to take a peek at is the function of the EGR valve if you still have one. Run a vacuum line separately down to the vacuum actuator on the valve and put a mouth vacuum on it until it retracts completely to check that there are no air leaks in the diaphragm. Quickly release the vacuum to allow the spring to bust any carbon that might have accumulated on the valve seat inside. Do that a couple times with the engine not running. The manual test for the EGR valve function is to start the engine and then apply a vacuum to the diaphragm at idle. The engine should stall. To test the vacuum modulator follow the test procedure in the book but a real quick check is to apply mouth pressure to the connection on the bottom. If there's a leak in the diaphragm inside the modulator, air will pass through. Otherwise, it should hold pressure if it's working properly. I had a problem with a hole in this diaphragm which caused the engine to run funny at speed with no load on. It felt like the engine was pulling and pushing. When I replaced the defective vacuum modulator, the problem went away.