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Discussion Starter #1
Hi, Sorry if this is to long below. I'm having a few issues with my SVO. I have a 84 mustang SVO, It has the 2.3l turbo motor. It has a misfire at idle. It also misfires when you rev it to 3500rpm or 4000rpm. it goes lean and back to rich on the Air/fuel gauge when it misfires. Its a random misfire. I tested the injector waveform with a scope and when it misfires it appears to drop on the waveform and return to normal randomly.

I can hear it misfiring/small popping in the exhaust at idle. the rpm moves up and down sometimes too. Any ideas?

I have good spark plugs and good wires. new cap and rotor and newer ford distributor.

My mechanic buddy thinks its either I have weak injector driver transistors in the ECM or it could be my injectors being to big since I have 35lbs brown top injectors instead of the 30lbs green top injectors that came in the 84-85 mustang SVO's. Could that be why I have a misfire? The inectors being the wrong pound injectors for that ECM? Causing the ECM injector driver transistors to slightly overload?

The injectors in this car are triggered by a ground circuit to open and close the injectors and they are batch fired.

My buddy thinks the injectors are spraying to much fuel which is causing it to go rich then the ecm trys to compensate for the rich condition and makes it go lean.

My ECM is the ZBA code.

The puse width on the scope says it is running at about 1.4 milliseconds to 1.7 milliseconds injector pulse on time. Not sure if that's where it should be or not.

Is the 84 computer compatible with the 35lbs injectors without modification?

The SVO's in 1984-1985 came with 30lbs injectors which where green top low impedance injectors.

The injectors that currently are in the car are 35lbs brown top injectors and they have 2.9ohms of resistance each when I test them individually with an ohm meter. Which are low impedance injectors.

I took the injectors out and used my custom made fuel rail and injector pulser to test the flow of the injectors each and they all seemed to flow good.

When I drive the car it doesn't have that much take off from the stoplight/bottom end but has a bunch of power on the top end when the turbo is kicking in. I'm wondering if this misfire would cause this?

Sounds like timing to me but the timing seems to be fine when I check it both cam timing and distributor timing which distributor timing is set at 10* Before TDC with spout disconnected.

Any help would be a big help. Been messing with this car for a while now and cant seem to pin point the issue.

Thanks
Jeff
 

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Is the 84 computer compatible with the 35lbs injectors without modification?

Yes, you should be able to run the larger injectors without an issue and the ECM normally will compensate. In some cases, it may be necessary to adjust the VAM via the mixture screw (located under a lead plug on the top of the VAM) or the internal adjustment on the air-door spring, which requires removal of the plastic cover atop of the VAM. This will only satisfy the idle and lower RPM range, roughly 3500 RPM or below.

What your describing sounds a bit like a lean condition, but your misfire could be caused by a variety of issues. I would check to see if fuel pressure is adequate under load 1rst. Your car would benefit a lot from a 255 LPH pump, if you don’t all ready have one.

The sluggish low end; is often a symptom of erosion of the inlet or exhaust impellers on the turbo or could be an issue of wear in the bearings. Back pressure, from an exhaust restriction will also slow down initial acceleration.

I have always favored more initial timing than the factory 10 degrees, even at the expense of lowering boost. You will find that 7-10 more degrees of initial advance will really wake your engine up on the low end and will make the engine much more responsive. If you run this much advance, keep an ear out for detonation.

Your friend could be right about the drivers, but usually these early Ford ECM’s are very tough and I've rarely seen them fail this way. I flat out disagree with his assessment on the injectors being too large. I have decades of experience fixing these cars which tells me otherwise.

Make sure that you don’t have a hidden vacuum leak somewhere and that the EGR valve is not stuck open.

The pulse width on the scope says it is running at about 1.4 milliseconds to 1.7 milliseconds injector pulse on time. Not sure if that's where it should be or not.
Pulse widths will be relative to engine speed and load; what you might notice with a good scope is an interruption or spike in the wave-form if you have a driver issue. Check for leaks and fuel pressure and let us know.
 

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I assume Ford plugs and wires and cap. The expensive platanium, split fire etc don't play well with the SVO engine. Measure fuel pressure with vac line off the regulator. Follow ohc230 good advice, check for vac leaks. Before I would condemn the ECM. I would check all the wiring from my reading the insulation on the early SVO goes bad.
Now another way to help is use a later computer like a LA series. Has a faster processor and better coding. Maurice
http://www.forced4.com/la3swap.html
 

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Discussion Starter #4
Yes, you should be able to run the larger injectors without an issue and the ECM normally will compensate. In some cases, it may be necessary to adjust the VAM via the mixture screw (located under a lead plug on the top of the VAM) or the internal adjustment on the air-door spring, which requires removal of the plastic cover atop of the VAM. This will only satisfy the idle and lower RPM range, roughly 3500 RPM or below.

What your describing sounds a bit like a lean condition, but your misfire could be caused by a variety of issues. I would check to see if fuel pressure is adequate under load 1rst. Your car would benefit a lot from a 255 LPH pump, if you don’t all ready have one.

The sluggish low end; is often a symptom of erosion of the inlet or exhaust impellers on the turbo or could be an issue of wear in the bearings. Back pressure, from an exhaust restriction will also slow down initial acceleration.

I have always favored more initial timing than the factory 10 degrees, even at the expense of lowering boost. You will find that 7-10 more degrees of initial advance will really wake your engine up on the low end and will make the engine much more responsive. If you run this much advance, keep an ear out for detonation.

Your friend could be right about the drivers, but usually these early Ford ECM’s are very tough and I've rarely seen them fail this way. I flat out disagree with his assessment on the injectors being too large. I have decades of experience fixing these cars which tells me otherwise.

Make sure that you don’t have a hidden vacuum leak somewhere and that the EGR valve is not stuck open.



Pulse widths will be relative to engine speed and load; what you might notice with a good scope is an interruption or spike in the wave-form if you have a driver issue. Check for leaks and fuel pressure and let us know.

I dont have the 255LPH fuel pump yet but I will eventually. my fuel pressure at idle is 37-38ish psi. The pressure regulator is working. I will check the fuel pressure under load tomorrow.

I replaced all the vacuum hoses. So there shouldn't be a vacuum leak. The EGR valve I will test tomorrow.

On the Snap-on Vantage scope I get a random spike in the waveform when it misfires. I have the scope red lead connected to the ground triggered side on the injector and the black lead of the scope on a known good ground like the ground on the battery.

When it misfires you can hear it in the exhaust and you can see it go all the way lean and then back to all the way rich on the air fuel gauge when it misfires as well.

So the things I will report back on are

1 - Fuel pressure under load.
2 - EGR functionality.
3 - Results on changing the timing to 7-10 more degrees.

Thanks!
Jeff
 

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Discussion Starter #5
I assume Ford plugs and wires and cap. The expensive platanium, split fire etc don't play well with the SVO engine. Measure fuel pressure with vac line off the regulator. Follow ohc230 good advice, check for vac leaks. Before I would condemn the ECM. I would check all the wiring from my reading the insulation on the early SVO goes bad.
Now another way to help is use a later computer like a LA series. Has a faster processor and better coding. Maurice
http://www.forced4.com/la3swap.html

My spark plugs are Motorcraft copper core plugs gapped at 28. My wires are the blue Ford Racing wires. I think they are 9mm wires. As for the cap and rotor I dont think they are Motorcraft brand but Im not sure. Maybe that's a problem? But I don't believe the misfire to be ignition related.

I have a fender mount MSD coil which looks just like the factory one except its red and black but I am not sure what model/line of MSD coils it is. I will check tomorrow and post back.

I have a newer Motorcraft distributor and Motorcraft TFI module installed in the car.

Ive repaired the bad wiring that I can find so far by soldering new wires in and also shrink tubing some of the bad wires.

Thanks for the replies.
Jeff
 

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Discussion Starter #6
Partial Test results

I dont have the 255LPH fuel pump yet but I will eventually. my fuel pressure at idle is 37-38ish psi. The pressure regulator is working. I will check the fuel pressure under load tomorrow.

I replaced all the vacuum hoses. So there shouldn't be a vacuum leak. The EGR valve I will test tomorrow.

On the Snap-on Vantage scope I get a random spike in the waveform when it misfires. I have the scope red lead connected to the ground triggered side on the injector and the black lead of the scope on a known good ground like the ground on the battery.

When it misfires you can hear it in the exhaust and you can see it go all the way lean and then back to all the way rich on the air fuel gauge when it misfires as well.

So the things I will report back on are

1 - Fuel pressure under load.
2 - EGR functionality.
3 - Results on changing the timing to 7-10 more degrees.

Thanks!
Jeff
So I had some time yesterday to do the EGR functionality test
and changing the timing to 7-10 more degrees. But I didn't have time to check my fuel pressure under a load. so I will get back to you on that when I test it.

The EGR seemed to work. But not sure how well. I applied vacuum to the port on the EGR valve and it stumbled but very little stumble. It stumbled hardly at all. I used a vacuum hand pump that has a vacuum gauge on it and the EGR valve didn't hold vacuum very long as it slowly came down to 0 on the gauge.

So I put a small BB in the vacuum line to the EGR valve to block it.

I tried to set the timing to 16 degrees before TDC. it idled better at 12-13 degrees before TDC. So I set it at 13* before TDC with the spout disconnected while adjusting and checking.

I do have a esslinger adjustable cam gear I think I forgot to mention I had. I've messed with the timing on the esslinger cam gear. I put it on TDC and put the cam on 0. Adjusted from there. I even took the valve cover off to verify the cam position by the lobs.

Thanks for the replies and help. Any more insight is welcome too.
Jeff
 

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Discussion Starter #7
When I say I put it on TDC I meant I put the crank on TDC adjust the cam to line up on 0 with the middle prong on the plastic timing cover lined up on the 0 of the cam gear. When crank is on TDC the distributor rotor is pointing to the #1 spark plug post.
 

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Jeff, it sounds like you've been very thorough with your diagnosis, but before you take a sledge hammer to the ECU, I might suggest that you carefully check one more time for loose grounds, corroded or burned wires. If you haven't done so, pull the plug from the ECU and make sure that you don't see any corrosion on the connector pins. There is also a remote chance you have a bad injector or 2, I've definitely had a few go bad.

Worst case scenario; I have a handful of "known working" turbo ECU's if you can't find one at a local Pick and Pull yard. Or if you know someone with a Merkur XR, SVO or 83-86 Turbocoupe, you could either fix the car or eliminate the ECU from the equation.

Wouldn't hurt to have a spare in any case.
 

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Discussion Starter #9
Jeff, it sounds like you've been very thorough with your diagnosis, but before you take a sledge hammer to the ECU, I might suggest that you carefully check one more time for loose grounds, corroded or burned wires. If you haven't done so, pull the plug from the ECU and make sure that you don't see any corrosion on the connector pins. There is also a remote chance you have a bad injector or 2, I've definitely had a few go bad.

Worst case scenario; I have a handful of "known working" turbo ECU's if you can't find one at a local Pick and Pull yard. Or if you know someone with a Merkur XR, SVO or 83-86 Turbocoupe, you could either fix the car or eliminate the ECU from the equation.

Wouldn't hurt to have a spare in any case.


I might be interested in one of your ECU's. I checked my local junk yards and they didn't have anything. How much would you want for one of your ECU's?

Do you have a 1987-1988 LA3 code ECU from a T-Bird by chance? If you have both the ZBA and LA3 label ECU's how much for them individually? I am aware of the repining the ECU connector for the LA3 computer.

I can get a reman ecm from orielly autoparts for $85 under my shop account.

One more question. My Dad said that if the exhaust has any pin holes in the welds or anywhere in the exhaust it can cause a misfire? any thoughts?

I do have a couple small pin holes in the exhaust welds. when the car misfires I get a small backfire in the exhaust. When this happens the A/F gauge goes lean then back to rich.

Thanks
Jeff
 
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