87 5.0 Troubleshooting & Solutions

Lateralgeez

Member
May 25, 2015
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In the engine bay of my 94 Miata lies an 87 5.0 of unknown mileage. In the 2 months of owning it I have already encountered a number of (expected) issues.

It had been sitting for 3 years when I bought it. First I added oil & coolant and new fuel filter. It would turn over but not start. With a new TPS my buddy adjusted it and the distributor until it ran.

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This worked for a few weeks until it started leaking coolant. Unfortunately it got up to 280 deg one day and blew rusty brown water all over the engine bay. The two front freeze plugs had rusted right through.

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With the new freeze plugs in everything was great. However, the guys on v8miata.net rightfully suggested a full engine and radiator flush. Two weeks later on the Dragon we noticed the passenger tail pipe smoking white under acceleration (Exhaust has no x or h pipe).

Three days later it developed a severe misfire - wouldn't idle without stalling and had little power. Regrettably I had to drive it like this for a few days until I got my other car fixed. Sometimes I could feel a cylinder come on while accelerating. I recognized this had been happening for a while before the misfires started.

So now the car is parked. Spark plugs are out and compression checked (dry on cold engine). Before I started driving the car, cyl 5 plug looked perfect. Now it's the worst with signs of sever heat including chipped ceramic.

By removing wires from the distributor while running we determined that cyl 2 and 6 weren't firing.
I checked to see if all the plug wires were separated and found some of them zip tied together! However, fixing this didn't change anything.

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Cyl 1
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Cyl 2
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Cyl 3
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Cyl 4
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Cyl 5
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Cyl 6
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Cyl 7
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Cyl 8
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This guide was helpful for troubleshooting BULLITT Archive Mustang Reading Your Spark Plugs

I have searched alot on google and this site. I believe a short list of things to check includes
timing (with a timing light)
injectors and o rings
spark plug wires
remove the heads and check valves

My biggest question is what (or what combination of things) could cause so much variation between the plugs. I noticed alot of spark plug questions on this forum don't include solutions so I plan to post whatever I find.
 
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Installed new Motorcraft spark plugs, still misfires
New Ford Racing spark plug wires, still misfires

However, it would idle... at 500 rpm lol. By touching the header tubes I determined that only cylinders firing were 1, 5, and 8. Yep that's right it is effectively a V3!
Pulling the plugs showed they were getting gas.
Using a screwdriver we checked for spark and got plenty of arc.
So fuel is good, spark is good, compression is good, that leaves air.

We sprayed maf cleaner around the intake manifold and the engine picked up noticeably. Next we removed the upper intake manifold. Almost all bolts on the lower intake manifold were loose!!!

After tightening the bolts and putting humpty dumpty back together we still had misfire :( BUT it was idling at 700 rpm! So not a total loss. Spraying maf cleaner indicated we still had a vacuum leak somewhere.

The conclusion is 1. that the lower intake gasket is shot or 2. the lower manifold warped when it overheated a few times.

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Yes we cleaned this after! Apparently it hasn't see daylight since 1987.

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Upper intake manifold gasket is good. Here is my buddy tightening the lower manifold bolts.

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The best advice I can give would be to rebuild the whole top end. It won't cost much and will answer a lot of questions. While you are there you may want to have the manifold planed for flatness. My suggestion would be to find a explorer intake in the junk yard.
 
I suspect a bad head gasket. Might not show up during a cold compression test. Once warmed up, are you noticing bubbles in your radiator, or coolant reservoir? Normally, I'd suggest adding some block tester fluid to the coolant to see if it's had CO2 mixed with it, but I'll assume the coolant is fresh since the frost plug change? It's sounding like it might be time for a top end rebuild as stated above....but I'd suggest running the codes and doing a cylinder balance test first.

And yes....a swap to a set of '95-'96 GT40 Heads and Intake will add an easy 40hp to your engine and for very little money. It would be a worth while mod while you've got the top end off.
 
Thanks for the help a91what and gearbanger! Coincidentally the mechanic that looked at it this afternoon suspected head gasket also. He said do a compression check with all spark plugs out.

The shop was about 5 miles down the road. Driving there I wasn't able to go over 30 mph on a long hill. Vacuum check was good so the lower manifold gasket isn't the problem. Something is wrong with the wiring because their scanner wouldn't connect to the ECU. During the testing they revved it up hard alot... and when I left it felt like it was down to 2 cylinders. The long hill now required 1st gear and barely moving. Finally it wouldn't go any further and my buddy towed me home! This sounds like the gasket is giving out.

a91what - I agree with you 100%, the top end is coming apart soon. Definitely going to check the heads and manifold for warp. It is an 87 speed density with the oem Mustang ECU.
 
I suspect a bad head gasket. Might not show up during a cold compression test. Once warmed up, are you noticing bubbles in your radiator, or coolant reservoir? Normally, I'd suggest adding some block tester fluid to the coolant to see if it's had CO2 mixed with it, but I'll assume the coolant is fresh since the frost plug change? It's sounding like it might be time for a top end rebuild as stated above....but I'd suggest running the codes and doing a cylinder balance test first.

And yes....a swap to a set of '95-'96 GT40 Heads and Intake will add an easy 40hp to your engine and for very little money. It would be a worth while mod while you've got the top end off.

Good suggestions! I'll try a warm comp check and look for bubbles. Coolant is going somewhere but not on the ground. The coolant is mostly fresh because I had to add a lot.
 
Regarding the GT40 heads and intake I found this:
"on stock E7 or GT40heads, no EFI intake will make as much torque and power below 4,000 rpm as a ported stock HO intake and it won't loose all that much up to about 5,200-5,500 rpm depending on the cam used." tmoss on corral.net

On the twisty roads around east TN i rarely see 5k rpm. So I'm sold on the exploder heads and porting the intake! Thankfully it's aluminum. Porting the iron exhaust manifold on my wrx sti was laborious so I'd rather just buy higher flowing heads.
 
Trust me when I say you will leave a lot on the table with the stock intake. Tmoss is right but I noticed gains across the board with my intake swap. The small runners on the stock intake do well with e7 heads because they increase port velocity, if you do the gt40 heads the stock intake will hold you back. And for the record every aftermarket intake is compared against the cobra/explorer intake. It still sets the standard for what a great street intake should be for the 5.0. My 2 cents.
 
Regarding the GT40 heads and intake I found this:
"on stock E7 or GT40heads, no EFI intake will make as much torque and power below 4,000 rpm as a ported stock HO intake and it won't loose all that much up to about 5,200-5,500 rpm depending on the cam used." tmoss on corral.net

On the twisty roads around east TN i rarely see 5k rpm. So I'm sold on the exploder heads and porting the intake! Thankfully it's aluminum. Porting the iron exhaust manifold on my wrx sti was laborious so I'd rather just buy higher flowing heads.
Porting the stock lower was the ticket before the aftermarket opened up and before the Explorer intakes were readily available. Now, you couldn't pay me to put one on there.

Just spend the $100 on the Explorer intake and don't look back. You'll never notice the oh so slight off idle torque advantage of the stock intake, but you'll sure miss what it leaves on the table up top.
 
Something is wrong with the wiring because their scanner wouldn't connect to the ECU. During the testing they revved it up hard alot... and when I left it felt like it was down to 2 cylinders.
.
Had I known that , I would have posted this...

Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs

Disconnect the battery positive terminal before making any resistance checks.
The voltage drop in the ground cable will cause incorrect resistance readings.


How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the computer into self test mode. If this ground is bad, none of the sensors mentioned will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. Since it is a dedicated ground, it passes through the computer on its way to the computer main power ground that terminates at the battery pigtail ground. It should read less than 1.5 ohms when measured from anyplace on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. Jumpering it to power can produce unknown results, including damage to the computer. The ohm test simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring :
With the power off, measure the resistance between the computer test ground (black/white wire) on the self test connector and battery ground. You should see less than 1.5 ohms.

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If that check fails, remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the black/white wire and pin 46 on the computer wiring connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. More that that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

See Computer issue? | Mustang Forums at StangNet for Joel5.0’s fix for the computer internal signal ground.

If the first ground check was good, there are other wires to check. Measure the resistance between the STI computer self test connector (red/white wire) and pin 48 on the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.
eec-iv-computer-connector-for-5-0-mustang-gif.88243


a9x-series-computer-connector-wire-side-view-gif.71316


Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the black/white wire goes. Almost every sensor on the engine except the MAF is connected to it.

88-91_5.0_EEC_Wiring_Diagram.gif


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info

See the graphic for the 10 pin connector circuit layout.
salt-pepper-10-pin-connectors-65-jpg.68512
 
Trust me when I say you will leave a lot on the table with the stock intake. Tmoss is right but I noticed gains across the board with my intake swap. The small runners on the stock intake do well with e7 heads because they increase port velocity, if you do the gt40 heads the stock intake will hold you back. And for the record every aftermarket intake is compared against the cobra/explorer intake. It still sets the standard for what a great street intake should be for the 5.0. My 2 cents.

Oh ok that clears it up then! I appreciate advise from personal experience.
 
Had I known that , I would have posted this...

Wow thank you! The wiring harness is kind of a mess. So what i need to do is

1. test black/white wire between self test and ecu for 1.5 ohms
2. test black/white wire and pin 46 on the computer wiring connector
3. test between the STI computer self test connector (red/white wire) and pin 48 on the computer main connector

I'll post the results!
 
Found out why the car wouldn't make it home from the shop... they left cyl 5 spark plug cover off! Der! Oh well.

I did another compression test. All the spark plugs were out which would reveal a head gasket leak between cylinders. Again cold and without adding oil. This time I kept the throttle open while I cranked 7 times each. The cylinders that are firing have the lowest compression! 1, 5, 8

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If both headgaskets are bad wouldn't it be much lower? Also, it should smoke terribly out both pipes. Currently it only smokes a little out the passenger side pipe under hard acceleration.

Next I tested the ECU - diagnostic ground wire for resistance (thanks stangnets favorite tool!) Turns out it was around 20 ohms! Seems strange this suddenly happened after running well for a few weeks.

I still plan on taking the top end apart to see what it looks like. Unfortunately electrical is a new can of worms for me. I'll take a look at the ground wire near all 5 sensors and get photos.

Worse comes to worse McCully racing makes a swap harness but it might require a MAF conversion.

See this work of art lol. The stang's ecu zip tied & taped to the dash :)

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In reality most of that mess is unnecessary. The engine harness and ecu need to be intact as well as ALL the grounds. I'm not sure how the rest of your harness connects together with the miata pieces. So some tidying up can be done. Once you start going thru it it starts to make more sense, I feel your pain tho I did this once to a 55 ford truck.
 
In reality most of that mess is unnecessary. The engine harness and ecu need to be intact as well as ALL the grounds. I'm not sure how the rest of your harness connects together with the miata pieces. So some tidying up can be done. Once you start going thru it it starts to make more sense, I feel your pain tho I did this once to a 55 ford truck.

Yeah its pretty ugly lol. I want to put ac, heat, and stereo in so all that mess might come in handy.
 
STI ground (black/white) to pin 46 stayed around 0.3 ohms. Apparently my test pin wasn't fully connected to the STI earlier which explains the unusually high reading. In other words, that ground wire is ok.
 
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