Surging/dying idle - Vintage Mustang Forums
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post #1 of 22 (permalink) Old 03-19-2018, 06:46 PM Thread Starter
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Surging/dying idle

Hello - I have the 5.0 from an 88 GT in my 1970 Mach 1. The engine runs fine but idles bigh when I first crank it and then idles down and fluctuates until it eventually shuts off. It doesn't happen every time. Sometimes it runs fine.

I have changed the O2 sensors and have changed the idle control valve, and it's still doing it. When I changed the ICV, I checked for carbon buildup and saw none, but still sprayed it down with brake clean just to be sure, but I am kind of at a loss at this point.

Does anyone have any ideas?
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post #2 of 22 (permalink) Old 03-19-2018, 10:32 PM
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Has it set a code? Do you have a MIL now? If so, is it on?
There's minimal data to be had but there's still some there if you have a an OBDI capable scanner that does data display. Things to look for would be like is the coolant temperature (ECT) is reading as expected cold and hot. Same with the IAT. Stuff like that rather than tossing parts at it.
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post #3 of 22 (permalink) Old 03-20-2018, 05:45 AM Thread Starter
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No, the MIL was never turned on when I bought the car, and we didnt do anything with it when we redid the gauges.

I have an OBDI reader that came with the car, but the previous owner nor I have been able to make it work, so that's my next purchase (should have been one of my first). I changed the idle control valve yesterday and drove it about 5 miles, and it was still cutting out on me and idling rough, but drove it 12 miles to work this morning and it was fine through all of the lights, no rough idle and didn't shut off on me. I will run by the parts store this afternoon and see if they have an ODBI. The one that came with the car is supposed to beep the codes, but I have a feeling I better get one with a display or I will never figure out what it's trying to tell me.
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post #4 of 22 (permalink) Old 03-20-2018, 07:18 PM
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I haven't had that problem on my 88gt but it does sound like a problem I had in my 86 bronco. That was the IAC valve. But since you changed it I'm not sure. I thought you weren't suppose to clean those with brake clean though.?

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post #5 of 22 (permalink) Old 03-20-2018, 08:21 PM Thread Starter
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I just cleaned out the carbon buildup in the housing, not in the new valve.

I just bought a new OBDI reader and it worked! I had several codes
34 - Insufficient EGR flow or EVP voltage high or PFE sensor voltage high)
81 - Air diverter solenoid fault, intake air control circuit fault
82 - Air diverter solenoid circuit failure/integrated relay control module
84 - EGR vacuum regulator circuit failure
And finally
85 - canister purge solenoid circuit failure/shift solenoid 3/4 - 4/3

Looks like I have some homework to do to figure all of this out, but it looks like I have somewhere to start now. All of these codes came with the engine off, but the ignition on.
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post #6 of 22 (permalink) Old 03-22-2018, 11:43 AM
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There is a wealth of information on this at
SURGING IDLE CHECKLIST-STANGNET

There are a great number of things that can cause it. I have issues with it on my 86 GT from time to time. The checklist at stangnet gives solid info and things to check, and solutions that some have found.

Ford even had a TSB related to idle surge, which they fixed with a adjustable idle plate to let some air bypass the IAC and fine tune idle speed. The TSB was to deal with sludge buildup causing idle/stalling issues, but the plate has been used many times to help the issue.
Attached Images
File Type: jpg tsb.jpg (103.6 KB, 6 views)

66 coupe- 302/T5, 3.80 gears, work in progress
86 GT- 5.0/T5 3.73 [email protected]
"If everything seems under control, you're just not going fast enough" - Mario Andretti

Last edited by v8stang289; 03-22-2018 at 11:50 AM.
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post #7 of 22 (permalink) Old 03-22-2018, 12:06 PM Thread Starter
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Thank you!
I warmed it up yesterday and hooked up the OBD1 to it while running, but one of the blades more or less fell apart, so back to the store I went. Went to another one, got the part, got home, same thing-warm it up, plug it up. No codes, so I decided to disconnect the battery and try again. Low and behold, my 6 month old positive battery terminal was in 2 pieces...Dig around, can't find another, so off to the parts store I go again. I finally got it installed at dark last night so hopefully today, I will be able to check the codes and get a full picture.
I am not running cats on it, so some of the codes I have I am not worried about, but hopefully I can get something meaningful today from the OBD1.
My other 70 mustang has none of this EFI/Emissions stuff as it has its original 351C, and is a whole lot easier for somebody as un-mechanically inclined as I am to work with!
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post #8 of 22 (permalink) Old 03-22-2018, 02:12 PM
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Your OBDI setup should be completely unaffected by whether you have cats or not. Lacking them should not set any codes, that came on later models.
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post #9 of 22 (permalink) Old 03-22-2018, 05:58 PM Thread Starter
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Well, that makes more sense. That's what I get for listening to a friend.

I ran it a few minutes ago, and still got the EGR codes in engine off and the air diverter solenoid codes. Then i cranked it up and got:
33 R -EGR valve not opening properly
41 R - HEGO sensor voltage low, system lean
91 R - HEGO Sensor voltage low/always lean

When I was finished, I went to crank it to move it into the garage and now the starter now won't disengage. Maybe the solenoid got fried, so I have to get that straightened out as well.

I am slowly learning about this car. It's gonna take a while. The convertible I have had for 28 years now is more or less bone stock and nowhere near as hard to work on.
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post #10 of 22 (permalink) Old 03-22-2018, 09:27 PM
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Blocking off the EGR is a popular thing to do with the 5.0 EFI guys even though it's actually slightly beneficial to street/daily driver performance and mileage. (The racers do it so it MUST be good!) So I'm assuming you've have something like that going on, hence the EGR code. Anyways if the EGR truly is not opening like the code says then it's having no effect on your issue. But if it were stuck OPEN then it certainly could cause not only your issues but the O2 codes too. As is, the O2 sensors reading like that would make looking for a big vacuum leak jump right to the top of my list.

Second on the list would be the temperature sensor that signals the ECM and the intake air temperature sensor. These can go wacky and conflict with each other or either can stick on one reading. They can do that and never set a code. Seen it too many times on OBDI vehicles. OBDII handles such temperature sensor failures much better.
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post #11 of 22 (permalink) Old 03-23-2018, 07:07 AM Thread Starter
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Thank you!

I am going to get the starter solenoid replaced today and will then tackle looking for vacuum leaks.

The temperature sensors being out of whack will be the next thing if I cant find a vacuum leak. I bet I have a leak somewhere. I looked last night and it looks like the EGR valve is new, and it looked like most of the vacuum lines were new. It very well could be that we knocked one loose when we were putting this thing back together.
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post #12 of 22 (permalink) Old 03-23-2018, 10:30 AM
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So you do have the EGR in place, that's good. The plastic line to it is usually green and goes back to a solenoid. Without vacuum the EGR can't open. If a vacuum line in that system is off it isn't helping for sure.
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post #13 of 22 (permalink) Old 03-27-2018, 06:02 PM Thread Starter
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Update - Got rid of the low voltage error codes by changing out the battery cable and fixed the starter with a new solenoid.
The car does not have an EGR valve. I didn't know what I was looking at the other night. The EGR has a blockoff plate over it.
I ran a smoke test on the vacuum lines while ago and we couldn't find any leaks coming from anywhere.
I ordered the engine temp sensor and a pcv valve from Rockauto and am going to try that.
I have Dakota digital gauges in the car and they came with their own gauge temp sensor. The stock gauge temp sensor is unplugged. Would that cause an issue?

I drove the car a few minutes ago and it did great for about 5 miles. Then, it shut off at a stoplight. I fired it right up and drove it on home. It wouldnt idle in the driveway, so what the heck, I hooked up the OBD1. It idled the car up went through the codes and still gave me the 33 R, 41 R, and 91 R codes. As soon as I turned off the OBD1 the car idled down and died....
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post #14 of 22 (permalink) Old 04-06-2018, 10:10 AM Thread Starter
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I figured out at least part of it - a vacuum leak at the brake booster. My wife spotted the tiny puff of smoke yesterday. I put a hose clamp on it, but I only had time to drive it enough to get it up to temp and then back home. Usually by the time I got home, it would die in the driveway. I was able to let it idle for about 5 minutes and it never died. I hope to be able to read the codes today.
I also found a questionable ground wire connection for the O2 sensor heaters. I had to take off the cowl cover to get the loose ground wire fixed, so hopefully, this afternoon will be the tell all....
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post #15 of 22 (permalink) Old 06-08-2018, 09:41 AM
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Hey 2-70's, I am dealing with something like this on my '65 with a 5.0. Did you ever figure it out?

Mine was running great and then one morning the car started fine and the idle raced up and down until it finally settled down to normal. I could drive it with the occasional stall and once warmed up would idle normally. I replaced the ECT sensor and it seemed to help for a while but over a period of weeks it got progressively worse. I replaced the IAC with no change and now it won't idle even when warmed up and stalls like yours was doing.

Jim Smith
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1965 Coupe 5.0 GT40 Intake and Heads, 65MM TB, chipped, T5, Shelby drop, roller idler, subframe connectors, Magnaflows, Hurst shifter, MS clutch, BFGs on 15" TTDs, Roller perches, Bilsteins, 3.54 gears, LS, Needs everything else...
"If you didn't build it; it's not really yours."
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