Annoying wavy idle and conk outs
12-04-2017, 08:21 AM
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Car: 1992 RS
Engine: 305
Transmission: A/T
Annoying wavy idle and conk outs
92 305 TBI
So Thursday I was driving home and I noticed a little coughing and hesitation when I would punch the accelerator while passing all the slow people. I didn't think much of it since at 260,000 miles a few things like that come and go sometimes. Friday morning it takes maybe an extra second of cranking and begins an annoying rhythmic wave of 1200-400 RPMs. Sometimes it dies and sometimes it powers through it. After maybe 5 minutes of this it levels out to a steady 1000, like, just suddenly levels out. It may drop to 900 or rise to 1100 occasionally but for the most part it sounds good and runs good, although still too high obviously. If I quickly rev it while still in park it may hesitate but it may not.
So I throw it into reverse and it dies. I start it again and rev through the conk-outs just to see how it drives. Other than the initial hesitation during wot acceleration it recovers nicely. It's not quite the same feeling as when my fuel pump was going out last year where the putting and coughing would continue through all stages of acceleration but would not affect the idle so I don't think and hope it's not that again.
My immediate thought was the IAC (original) or TPS (Autozone lifetime) so I replaced both and cleaned the IAC port. That seemed to help a little but people always say that though don't they? Granted it was 75 degrees outside in the afternoon when I had nothing to do but work on my car instead of 55 degrees in the morning when I actually needed to go to work which seems to be a relevant factor. The strange part is that after changing them, the cold start idled fine for about 5 minutes and then the issue came back for a minute or so and then went away again. (still idled high though)
I've visually inspected both sides of every vacuum hose and they are in good shape overall but admittedly I have not inspected every inch of every hose yet. My full tuneup + O2 sensor is about due so I'm going to do that anyway. I did pull codes 42 and 44 but I have not had an SES light come on since this started Thursday so while they may be relevant I would have thought they'd be lighting up during this actual problem. My main confusion comes from the intermittent wave. I would think if it was vacuum related it would always do it and if it was a sensor or valve it would trip a code while it was happening.
Fast forward to this morning and bam! it's back and worse than ever. Since my round trip is about 80 miles I wasn't going to be foolish enough to try it so I'll spend the day working on this. This is more of a placeholder for my frustration while I troubleshoot more but please feel free to make suggestions.
Too long didn't read version:
400-1200 rhythmic wavy idle that comes in spurts of 1-5 minutes
hesitant wot acceleration
new IAC and TPS didn't fix it
fuel pump and filter a year old
vacuum seems ok
codes 42 and 44 but not since or during this issue
presents itself fully when I need to go to work
So Thursday I was driving home and I noticed a little coughing and hesitation when I would punch the accelerator while passing all the slow people. I didn't think much of it since at 260,000 miles a few things like that come and go sometimes. Friday morning it takes maybe an extra second of cranking and begins an annoying rhythmic wave of 1200-400 RPMs. Sometimes it dies and sometimes it powers through it. After maybe 5 minutes of this it levels out to a steady 1000, like, just suddenly levels out. It may drop to 900 or rise to 1100 occasionally but for the most part it sounds good and runs good, although still too high obviously. If I quickly rev it while still in park it may hesitate but it may not.
So I throw it into reverse and it dies. I start it again and rev through the conk-outs just to see how it drives. Other than the initial hesitation during wot acceleration it recovers nicely. It's not quite the same feeling as when my fuel pump was going out last year where the putting and coughing would continue through all stages of acceleration but would not affect the idle so I don't think and hope it's not that again.
My immediate thought was the IAC (original) or TPS (Autozone lifetime) so I replaced both and cleaned the IAC port. That seemed to help a little but people always say that though don't they? Granted it was 75 degrees outside in the afternoon when I had nothing to do but work on my car instead of 55 degrees in the morning when I actually needed to go to work which seems to be a relevant factor. The strange part is that after changing them, the cold start idled fine for about 5 minutes and then the issue came back for a minute or so and then went away again. (still idled high though)
I've visually inspected both sides of every vacuum hose and they are in good shape overall but admittedly I have not inspected every inch of every hose yet. My full tuneup + O2 sensor is about due so I'm going to do that anyway. I did pull codes 42 and 44 but I have not had an SES light come on since this started Thursday so while they may be relevant I would have thought they'd be lighting up during this actual problem. My main confusion comes from the intermittent wave. I would think if it was vacuum related it would always do it and if it was a sensor or valve it would trip a code while it was happening.
Fast forward to this morning and bam! it's back and worse than ever. Since my round trip is about 80 miles I wasn't going to be foolish enough to try it so I'll spend the day working on this. This is more of a placeholder for my frustration while I troubleshoot more but please feel free to make suggestions.
Too long didn't read version:
400-1200 rhythmic wavy idle that comes in spurts of 1-5 minutes
hesitant wot acceleration
new IAC and TPS didn't fix it
fuel pump and filter a year old
vacuum seems ok
codes 42 and 44 but not since or during this issue
presents itself fully when I need to go to work
12-04-2017, 10:24 AM
#2
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Join Date: Jul 1999
Location: Denton, TX
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Car: 1992 RS
Engine: 305
Transmission: A/T
Re: Annoying wavy idle and conk outs
Ok I found that way too easily. I blocked off the MAP sensor vacuum port on the TBI and started it. It really didn't want to stay started for about 2 seconds then it smoothed out like butter at 1100 RPM. I let it warm up for about 5 minutes and the idle only got better. Settled around 1000 with no stuttering whatsoever. I gave it about 10 quick revs with not the slightest hesitation.
Hooked the MAP back up and all the bad stuff was back. Rough idle and hesitation when I stomped the gas. I repeated this about 6 times with and without MAP and the results were 100%.
So now, is it the MAP or the wiring and if it's the MAP will it drive ok until I get one? I saw quite a few posts about people never actually seeing a bad MAP so I was like, why even check that?
Hooked the MAP back up and all the bad stuff was back. Rough idle and hesitation when I stomped the gas. I repeated this about 6 times with and without MAP and the results were 100%.
So now, is it the MAP or the wiring and if it's the MAP will it drive ok until I get one? I saw quite a few posts about people never actually seeing a bad MAP so I was like, why even check that?
12-04-2017, 12:44 PM
#3
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Join Date: Jul 1999
Location: Denton, TX
Posts: 120
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Car: 1992 RS
Engine: 305
Transmission: A/T
Re: Annoying wavy idle and conk outs
A quick update before anyone responds... 
As smooth as it runs with the MAP disconnected and port blocked off, apparently the computer doesn't like reverse or drive because it stalls almost immediately. Hopefully this is expected and I just have to wait for the new MAP to arrive. I was going to chance driving it for 1 day MAPless but I guess that's not an option now unless anyone knows any tricks.
As smooth as it runs with the MAP disconnected and port blocked off, apparently the computer doesn't like reverse or drive because it stalls almost immediately. Hopefully this is expected and I just have to wait for the new MAP to arrive. I was going to chance driving it for 1 day MAPless but I guess that's not an option now unless anyone knows any tricks.