Intermittent miss...

[Boydster]

OK. Got the following problem:
Engine cold or hot, I have what seems like an intermittent ignition miss. It will do it 2 times in 5 seconds or 1 time per minute or 3 times in 5 minutes... you know what I mean. It's a very fast, split second miss, like just one spark on one cylinder is dropping off. I might see the tach drop 200 rpm, but just for that split second. Does it idling or revving. Have only driven the car once since the rebuild, and it did it 3 times around the block, mostly on acceleration. I can simply idle it in the driveway and it'll do it.

It's a 1987 with the FMS Mass Air Conversion, A9S ECU. I know, California, yuck, but it's what I've got.

I have an Innovate LC-1 installed in the LH downtube, and when it misses, I can see the AFR go lean for a few seconds. My thinking is that the ECU O2 is seeing rich for a second due to the missfire, and is leaning that bank. After just a few seconds, it goes right back to running about mid 13's to low 14's.

Been doing this since the rebuild up to a 306. New MSD Pro Billet was installed at engine build, timed to 10*, with new TFI and PIP, cap and rotor. New plugs (Champion RC12YC for the Edelbrock heads), FRPP 9mm wires, Accel EFI coil. Had originally bought CList Cobra 24lb'ers with adapters for the rebuild, replaced with new Accel 24's. C&L 73mm MAF with a 24 tube. Cleaned and inspected salt n peppers, TFI plug, spout plug. All this no help.

I've pulled KOEO, KOER codes with an Actron... all I get is emissions related, which has all been properly removed. For the record, I get a KOEO 84 and 95, with a 95 in memory. I understand the 95 can be Thermactor or FP Monitor, but since the engine starts and runs great except for the miss, and all the Therm stuff is gone, I'd say it's not FPM. KOER I get 94, 44 and 33... all emissions.

Guess I could go out and spend another $50 on a TFI and PIP, but was wondering if there were any other ideas that made more sense. Understand both of these will normally cause a shutdown, not just an intermittent miss.

Thanks StangNet... any q's that I didn't cover, feel free to ask. More info about the engine in my sig...

 

[Noobz347]

The problem could very well lie in ANY of those ignition related items that you listed above.

Were it me though (and we all spit ball differently), I'd replace the cap (possibly cracked) followed by the plug wires.

Once upon a time I had the same issue that you're having. the funny thing is that I was using the FMS wires that you're using. After chasing this thing down for a while, I went with a set of Taylor plug wires. Not so much as a hiccup since.

Ignition was jumping to ground through the wires... it ended up being more than one even. FMS wires=Never again.

 

[Boydster]

Thanks Noobz. Maybe I didn't spell it all out exactly right... post was gettin kinda long.

The wires, plugs and coil were all replaced after this problem started, and did not help. It had 20 yr old FMS wires, recent plugs and an old design (non plug-in... used jumper wires) Accel coil.

But I'll keep the idea of using a different manufacturer on the wires...

 

[jrichker]

Cylinder balance test:

Revised 30-July-2011. Added need to make sure the clutch is pressed when dumping codes.

The computer has a cylinder balance test that helps locate cylinder with low power output. You’ll need to dump the codes out of the computer and make sure that you have the A/C off, clutch depressed to the floor and the transmission in neutral. Fail to do this and you can’t do the engine running dump codes test that allows you to do the cylinder balance test.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C clutch depressed to the floor, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.


Here's how to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Cylinder balance test
Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test.The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure


Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

See the link to my site for details on how to build your own blow down type compression tester.

 

[Boydster]

Thanks, Jrichter. I don't think I have a compression problem, but I suppose this could help isolate the issue. I'll report back with what I find out...

 

[Noobz347]

Thanks, Jrichter. I don't think I have a compression problem, but I suppose this could help isolate the issue. I'll report back with what I find out...

Your post made me think of it because I did the very thing you did. I replaced and old set wires with a BRAND NEW set of FMS wires. In the end, swapping out those FMS wires for the Taylor set, fixed my problem. That's why I say NEVER AGAIN with the FMS wires. They were brand spankin new and I was pulling my hair out. I BOUGHT them to fix the very problem that they ended up not fixing. hehe

 

[Boydster]

Ah, Gotcha. I'll keep that in mind as a likely suspect.

 

[Boydster]

Your post made me think of it because I did the very thing you did. I replaced and old set wires with a BRAND NEW set of FMS wires. In the end, swapping out those FMS wires for the Taylor set, fixed my problem. That's why I say NEVER AGAIN with the FMS wires. They were brand spankin new and I was pulling my hair out. I BOUGHT them to fix the very problem that they ended up not fixing. hehe

Hey Noobz, just an update... changed out the TFI and PIP, no help. Ordered a set of Taylor wires today... we'll see how that does. If that doesn't do it... well, I just don't know anymore.

 

[Noobz347]

Well I hope it works out for you as well as it did for me. I chased the damned thing around pulling my hair out for a couple of weeks.

I discovered the problem by sheer luck. I had popped the hood with engine running... pitch black outside, and saw the faintest arcs coming from various places in the wires.

This after having changed coil, wires, cap, rotor, TFI, everything BUT the dizzy and PiP (which were going to be next). I was even troubleshooting wires to/from the dizzy and coil.

Hope that fixes ya. Let me know.

 

[Boydster]

Hope that fixes ya. Let me know.

Grrrrrrr.... no help. But I do think the 409 Taylors did make it run a bit smoother. Hard to tell with a TFS-1.

Pulled codes again. Nothing except the emissions stuff that has been removed... 95, 33, 44. Cylinder balance test shows a "9"... All OK.

I'm about ready to slap a 6AL on it and see if that will cover it up. Everything in the ignition has been replaced.

 

[Noobz347]

I'm beginning to wonder if there's a problem with your fuel injection harness or an injector. You said above that when it stumbles, your AFR goes LEAN. That's not typical for an ignition miss. Usually a miss will cause AFR to go rich before it straitens itself out.

 

[Jason 302]

Years ago I put a Auto Zone cap and rotor on my car and had issues similar to yours. As soon I put the stock pieces back on it quit.

 

[Boydster]

Years ago I put a Auto Zone cap and rotor on my car and had issues similar to yours. As soon I put the stock pieces back on it quit.

Thanks Jason. I have a brand new copper contact MSD cap and rotor on top of a new Pro-Billet distributor. I've checked inside, and there's no carbon tracking or evidence of arcing.

Guess I'm going to have to look into other systems, but not sure where to go. Tank has been cleaned out with a gallon of acetone, inspected (very clean inside), new fuel pump sock, new filter, new injectors...

If anyone has any other ideas, I'm listening...

 

[Boydster]

Update: Replaced the TFI and PIP, no help. Replaced fuel filter, no help. Changed timing from 10 to 12 to 14 and back to 10, no help.

Just ordered a new computer, 78-5611 (Cardone rebuild, A/T, non-Calif, Mass Air) from O'Reilly. Yeah, I know I'm tossing parts at it, but with no codes outside of emissions, good cyl drop test and everything else has been replaced... I'm at a loss.

Can't drive it like this... hate to start parting it out after 24 years.

 

[Boydster]

I'm beginning to wonder if there's a problem with your fuel injection harness or an injector. You said above that when it stumbles, your AFR goes LEAN. That's not typical for an ignition miss. Usually a miss will cause AFR to go rich before it straitens itself out.

Sorry Noobz, just now saw this. I've already replaced the injectors since this started, with no help. Just ordered a new computer... if that doesn't fix it, I'll look into replacing the injector harnesses. Although I looked em over pretty good during the rebuild.

I'm wondering about the ground shielding that wraps the wiring going to the TFI and the shorting plug. Maybe if that ground was bad, it could be picking up some inducted interference...

 

[Noobz347]

Sorry Noobz, just now saw this. I've already replaced the injectors since this started, with no help. Just ordered a new computer... if that doesn't fix it, I'll look into replacing the injector harnesses. Although I looked em over pretty good during the rebuild.

I'm wondering about the ground shielding that wraps the wiring going to the TFI and the shorting plug. Maybe if that ground was bad, it could be picking up some inducted interference...

Ok... now I'm just shooting from the hip but this suggestion won't cost you anything...

Salt and Pepper shakers... pull em, clean em, check all the pins for proper seat and reinstall.

 

[Boydster]

Salt and Pepper shakers... pull em, clean em, check all the pins for proper seat and reinstall.

Already done. None are loose, no corrosion, cleaned em with DeOxit Gold. Also did the shorting plug & TFI plug. Thanks for keepin on trying, man. I appreciate it.

 

[Noobz347]

Already done. None are loose, no corrosion, cleaned em with DeOxit Gold. Also did the shorting plug & TFI plug. Thanks for keepin on trying, man. I appreciate it.

That Deoxit is no joke. I'd say you've done all that can be done there.

Sure am interested in finding out the eventual cause.

 

[Boydster]

That Deoxit is no joke.

Perks of the job.

Computer will be here about Friday. I'm gonna check that shielding and the ground for the ignition wiring in the meantime.

 

[No.11]

could it possibly be a weak fuel pump causing it? have you checked fuel pressure?