87 gt t5 swap idle issues

noah139

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Jun 26, 2022
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I have a 1987 GT that I did a t5 swap on a year ago. I've been dealing with an idle issue. It started off with a major jump from 400 to 1700 non stop. I messed with the idle and got it to be more stable but still jumped around. I swapped the ecu to a manual transmission ecu and that made the car drivable. I followed the lmr video online when doing the whole swap. I got a used t5 and when i put it in I found out the speedo did not work. So as of right now my speed sensor is not plugged in. I was going to pull the trans and either rebuild or replace it. My main concern is will fixing the speedo fix my idle because I have heard many mixed answers on what the speed sensor is actually for. Any advice helps
 
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Mustang5L5

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did you swap the transmission harness out to the manual harness and connect the NGS on top of the T5?

As a work-around, disconnect the clutch NGS and install a 5amp blade fuse. See if that corrects your idle issues. If it does, then you need to troubleshoot the NGS circuit (which tells the ECU the car is in nuetral so it can idle better)

If you dump codes, do you have a code 67?

Is the car mass air swapped? It is speed density, the speed sensor won’t matter. The 87-88 cars only use the speed sensor for cruise control.

If the car is mass air swapped, tou need to add wires to connect the ECU to the speed sensor as they are not present in the standard 87-88 wiring.
 

noah139

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Jun 26, 2022
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did you swap the transmission harness out to the manual harness and connect the NGS on top of the T5?

As a work-around, disconnect the clutch NGS and install a 5amp blade fuse. See if that corrects your idle issues. If it does, then you need to troubleshoot the NGS circuit (which tells the ECU the car is in nuetral so it can idle better)

If you dump codes, do you have a code 67?

Is the car mass air swapped? It is speed density, the speed sensor won’t matter. The 87-88 cars only use the speed sensor for cruise control.

If the car is mass air swapped, tou need to add wires to connect the ECU to the speed sensor as they are not present in the standard 87-88 wiring.
I did try the 5 amp trick and it didn’t really do anything. The car is not mass air equipped. Lmr also mention the clutch dump switch but I did not hook that up cause I couldn’t find it.
 

Mustang5L5

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Is the car is not mass air swapped don’t worry about the speed sensor. It’s only used for the cruise control.

Did you hook up the switch on top of the T5?


You said you did the 5A fuse trick. Where did you plug it in? Did you connect it to the clear plug next to the clutch switch? Did you plug these two plugs into the switch? One is the starter safety plug

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noah139

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Is the car is not mass air swapped don’t worry about the speed sensor. It’s only used for the cruise control.

Did you hook up the switch on top of the T5?


You said you did the 5A fuse trick. Where did you plug it in? Did you connect it to the clear plug next to the clutch switch? Did you plug these two plugs into the switch? One is the starter safety plug

74986FBA-1B9E-4E96-BDEB-05F225C1BD53.jpeg
B79E7A6F-21EF-42AD-B254-B5AF298191C5.jpeg
I connected it to the clear plug. I’m not arguing with you guys but I’ve heard multiple times that the speed sensors also tells the computer to raise the idle once the car gets below a certain mph. I’m leaning towards getting a manual trans harness when I do the trans. I just don’t want to do all that work and still have the idle issues. It is tolerable until I turn the ac on. The the idle has a stroke
 

Mustang5L5

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And I agree with those folks. The speed sensor does affect idle quality, but only on the 1989+ cars with mass air. If you had done a mass air conversion I would absolutely be telling you that it needs to be hooked up.


Since you are factory speed density, it’s not a factor. Ford doesn’t even install the speed sensor on 86-88 cars that didn’t have Cruise control. Your problem lies elsewhere
 

noah139

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Jun 26, 2022
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And I agree with those folks. The speed sensor does affect idle quality, but only on the 1989+ cars with mass air. If you had done a mass air conversion I would absolutely be telling you that it needs to be hooked up.


Since you are factory speed density, it’s not a factor. Ford doesn’t even install the speed sensor on 86-88 cars that didn’t have Cruise control. Your problem lies elsewhere
Ok so now we may have gotten somewhere. The computer I put in was from a 89 I believe. Should I get a manual trans harness and put the stock computer back in?
 

KRUISR

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I did the T5 swap thing last year at this time. I'm a 90 mass air car. I kept the A9P (auto) computer and made no changes to the wiring. I did not connect the clutch pedal safety switch and kept the jumpers in place that an auto car comes with. I did jumper the NSS switch that came from the automatic because my T5 did not have one. At the time I had an issue with the speedo gear and did not have the speed sensor functional. I did a full base idle reset and have had no issues. This spring I swapped a 94GT T5 for the V6 T5 I had put in last year. I pulled the tail shaft on the GT trans to swap to a 7 tooth drive gear, now my speedo is good.

Because I still have the auto computer, when I am slowing down for a stop and put it in neutral to coast, the car will idle at 1200-1300 (steady) until I stop and then drop to 800 rpm or so - immediately as I stop moving. I have been told this is a function built into the auto computer for coasting down.
 

Mustang5L5

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Ok so now we may have gotten somewhere. The computer I put in was from a 89 I believe. Should I get a manual trans harness and put the stock computer back in?


Can you post a pic of the label on the ECU?

You can’t swap a mass air ECU into a speed density car without repinning the ECU harness a bit and adding the MAF.
 

noah139

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Jun 26, 2022
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I drove the car yesterday and that was after disconnecting the batter so I believe the computer reset. It drove perfectly fine the entire time until I turned the ac on and that made the idle bounce around from then on out.
 

jrichker

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Computer & interchange information.
The Mass Air computers from 89-93 are interchangeable with a few exceptions. Just be sure you use the MAF & MAF sensor from the same series computer. The auto transmission computers use a different start circuit than the manual computers. You can use an auto trans computer in a 5 speed car with no problems or changes.I have done it in my original 89 Mustang GT, and I know it works OK. I have no positive input or comments on using a 5 speed computer in an auto trans car. I personally would not do it.

The odd duck is the 93 Cobra computer, labeled X3Z which is internally calibrated for 24 lb. injectors. Use the MAF & MAF sensor from the X3Z computer to avoid problems with the computer's internal calibration. Try and avoid the 93 Cobra computer if you can.

Some cautions about O2 Sensor harnesses need to be observed to avoid damage to the computer.
Only run a auto trans O2 harness with an A9P
Only run a 5 speed trans O2 harness with an A9L
See http://forums.corral.net/forums/gen...manual-auto-differences-year-differences.html for more O2 sensor wiring harness info

All the following are mass air unless they say Speed Density

8LD - MANUAL 88 5.0L Mustang Speed Density
8LF - Auto 88 5.0L Mustang Speed Density
A9L - 89-93 5.0L Mustang 5-spd Mass Air
A3M - 93 5.0L Mustang 5-spd Mass Air
A3M1 - 93 5.0L Mustang 5-spd Mass Air
X3Z - 93 5.0L Cobra 5-spd Mass Air internally calibrated for 24 lb injector
A9P - 89-93 5.0L Mustang Auto Mass Air
A9S - 88-93 5.0L Mustang California Mass Air
C3W - 89-93 5.0L Mustang Auto
DA1 - 87 5.0L Mustang 5-spd Speed Density
J4J1 - 94-95 5.0L SVT Cobra 5-spd
T4M0 - 94-95 5.0L GT Vert 5-spd
U4P0 - 94-95 5.0L GT Auto
W4H0 - 94-95 5.0L GT
ZA0 - 95 Cobra R
D9S - Lincoln LSC

IF you have a Mass air computer, then the following tech note applies...

MASS air conversion instructions from http://www.stangnet.com/tech/maf/massairconversion.html FREE

Revised 2 Aug 2014 to add new links to conversion harness suppliers, diagram for repining computer wiring connector and YouTube video

A9L (5 Speed) computer from junkyard $100-$150 -maybe more – A9L computers are becoming hard to find.
A3M (5 Speed) computer from junkyard $100-$150
A9P (Auto or in a pinch, it will work in a 5 Speed car) computer from junkyard $100-$150
70MM MAF from 94-95 Mustang GT - $40-$70
MASS Air wiring harness kit $30-$85

The whole thing is probably less than $300 using junkyard parts.

A9L computers are 5 speed only
A9P computers are automatic, but will work with a 5 speed.

The conversion harness seems to work well for most folks. It avoids the compatibility problems in using a harness from the junkyard. Simple and cheap, actually less work that swapping the wiring harness.
Conversion Harness kits & parts

Youtube video how to do a Mass Air conversion using a conversion harness kit (somewhat long but useful)

View: http://www.youtube.com/watch?v=mnnTgXkHU9Q


hrdp_0705_26_z-ford_thunderbird_harness-.jpg


Wire side 60 pin computer wiring harness connector
a9x-series-computer-connector-wire-side-view-gif.71316


Computer side 60 pin computer wiring harness connector
eec-iv-computer-connector-for-5-0-mustang-gif.88243



If the idea of moving & soldering wires scares you, here's a list of compatible Mass Air wiring harnesses.

Copied from bbunt302
Just for reference, here's a list of all the compatible years:

89 harness should work for 86-89 as long as you're using mass air.
90 harness will only work in a 90. (because of air bags and dual dash connectors)
91 through early 92 harnesses should be compatible (single dash connector, fuel pump relay under driver's seat)

Late 92 through 93 harnesses may be compatible (single dash connector, fuel pump relay under the hood). You may end up running some extra wire for the fuel pump relay. Comments and input from someone who has actually used the 92-93 EFI harness would be nice.

Larger MAF to go with Mass Air conversion:
94-95 Mustang GT MAF - $40-$100. It is 70 MM instead of the stock 55 MM on regular stangs built prior to 94. It uses a slip on duct on the side that goes to the throttle body and a 4 bolt flange on the other. You need a flange adapter to fit the stock slip on air ducting that goes to the air box. Wiring plugs right in with no changes. *1 *2

*1.) Metal flange adapter http://www.kustomz.com/cat3.html Buy the TR70 for $40-$45 depending on if it’s on sale or not. Or spend some time on eBay looking for one that may fit.
Try AutoZone and ask for 81413 - Spectre / 3 in. Aluminum Intake Mass Air Flow Sensor Adapter at $12.00. You may have to order it online.

*2.) MAF & sensor interchange
The 94-95 Mustang 5.0 MAF & sensor is also found on:
1995-94 Mustang 3.8L F2VF-12B579-A2A,
1994-92 Crown Victoria 4.6L F2VF-12B579-A2A,
1995-94 Mustang, Mustang Cobra 5.0L F2VF-12B579-A2A,
1994-92 Town Car 4.6L F2VF-12B579-A2A,
1994-92 Grand Marquis 4.6L F2VF-12B579-A2A,
Evidently the –A1A, -A2A, AA, etc. on the end of the part number is a minor variant that did not change the operating specs. You should be able to ignore it and have everything work good.



Once you have done all the work previously mentioned items, then...

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
 
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AeroCoupe

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So just out of curiosity and to wrap all this up was the car converted over to Mass Air? Just asking as a mass air ECU will not work in a speed density car without repinning the wiring harness and adding the mass air wiring and what not.