Anyone experience a rapid drop in rpm when downshifting or stopping? My tach drops to 500 rpm when downshifting then jumps back to to 900 normal idle ,does not cut off I have a newer tps. Just seeing if this is normal or somepart I have to change? I have a 86 2.8 4 speed
If it's a true RPM drop and not just the tach acting up, might be a slow Idle Air Control motor. Maybe some gunk built up in the passage. Cleaned out the IAC passage lately?
Well the rpms rapidly will go to 500 and jump right back about to 900 idle speed i just think its strange that they have to go down that much considering my idle rpm is 900. The car has never cut off on me since I replaced the tps sensor.
[This message has been edited by Atlanta1 (edited 08-14-2009).]
Well the rpms rapidly will go to 500 and jump right back about to 900 idle speed i just think its strange that they have to go down that much considering my idle rpm is 900. The car has never cut off on me since I replaced the tps sensor.
Ah... huh, yup, no idea :P
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10:32 PM
Indiana_resto_guy Member
Posts: 7158 From: Shelbyville, IN USA Registered: Jul 2000
Check the throttle body where the butterfly comes to rest at idle position, there can be a small build up of carbon and gunk at that point choking the air supply off. There are also two small hole near the 6 o'clock position that supply a small amount of air to the intake passage that help that problem, check those and make sure they are open - - - same location.
I just bought a new iac valve from pep boys for $22.00 made by borg warner they gave me a price match. I will have it installed on Wednesday. Also I discovered while I was driving my car on a cold engine I did not see the rpm do the rapid drop to 500rpm and jump back up must be something to do with the IAC valve. My volts also go down to 12 and go back up when slowing down quick must be the Power pulley doing that or it can be related .
[This message has been edited by Atlanta1 (edited 08-18-2009).]
Well Folks the results are in... The new IAC valve did not improve the low idle drop in my car when slowing with clutch in or downshifting. I will run a fiero scan and see if I can post the results. It must have something to do after the car warms up because on a cold engine I dont have this issue. Any ideas are welcome
Lightly touch (Don't rev...) the throttle when the idle drops, and see if it springs right back. Try to notice what RPMS you're at when you go to stop. You might want to check how smooth the travel on your throttle body is, including your new TPS. I've had these things be garbage brand new out of the box. (These things are like old volume knobs in stereos. They get worn at the idle spot usually, but people who do a lot of highway driving wear a sweet spot from cruising at consistent speeds) It sounds to me like something is sticking, and catching up within a couple of seconds.
Ok folks I now have tried taping the accelerator a little when I put the car in nuetral to slow down and the rpm did not respond However I did hear my belt squeal in 2 occasions. I have installed a Rodney Dickman power pulley that also squeals when I do a quick downshift from 3rd to 2nd. The car at a stop will bounce back from 500rpm to 900 idle speed. This rapid drop in rpm to 500 is happening 70% of the time. So if the belt squeals a little when tapping the accelerator and the rpms dont readily respond is this going back to a bad tps? What I need to do is run a fiero scan and post the results. Also when the car is not in operating temp I dont have the rapid rpm drop going on. Anyone want to comment please feel to share your thoughts.
Ok folks I now have tried taping the accelerator a little when I put the car in nuetral to slow down and the rpm did not respond However I did hear my belt squeal in 2 occasions. I have installed a Rodney Dickman power pulley that also squeals when I do a quick downshift from 3rd to 2nd. The car at a stop will bounce back from 500rpm to 900 idle speed. This rapid drop in rpm to 500 is happening 70% of the time. So if the belt squeals a little when tapping the accelerator and the rpms dont readily respond is this going back to a bad tps? What I need to do is run a fiero scan and post the results. Also when the car is not in operating temp I dont have the rapid rpm drop going on. Anyone want to comment please feel to share your thoughts.
Check your tension on the alternator belt... Probably unrelated, but a squealing belt means slipping. May as well fix it. (A little belt dressing after re-tensioning the belt would be a good idea. Would really need to hook a multimeter to your TPS to know if it's the problem. I look to the TPS as a natural tendency because it is a moving electrical part that often goes bad with these types of symptoms. But I wouldn't start swapping parts without figuring out the cause. Part swapping will obviously eventually fix anything when you get to the right part; but it get$ co$tly quickly. Do you have a decent volt / ohm meter? (Checking your TPS with a multimeter will eliminate it from the equation). The fact that this problem changes with operating temp is what puzzles me. Mass Air Flow sensors would cause some engines to perform poorly in the cold, but you're saying the problem occurs when the engine reaches normal temps. Will have to sleep on this. Very puzzling.
Everyone New results. I ran my fiero scan and found a HIGH VOLTAGE MAP SENSOR code. I always had a speed sensor code that will light up every now and then. I did not realize that a map sensor code was a problem since I did not see a continuous orange light on my dash.(kit) My MAP sensor registred a 1. 40 volt average. My tps show volts .92 idle to to 1.24 on rpm around 2500 So I have a vss sensor code and a Map sensor high voltage. My Vss sensor was changed out about 7,000 miles ago I dont think that has anything to do with my rapid idle drop. My IAC step show from 16 inidle to a high at 29 around 2800rpm Feed back would be greatly appreciated I will have to replace the Map sensor
[This message has been edited by Atlanta1 (edited 08-21-2009).]
Just looked at my records I replaced the Map sensor around 4,000 miles ago must be a bad part let me know if you need any more results from my Fiero scan. Just wandering now if the bad Map sensor is causing most of my rpm drop issues
