Chasing Misfire With New Symptoms

Htxgen · Jun 19, 2017

hey guys, I have an old thread on here about rough idle that seems to be a misfire. I'll keep this as short as possible by hitting my issues quickly.

2000 mustang 3.8 automatic - 87k miles

Original issues

Slow crank on cold start but always will start

Rough idle that can get worse in heavy heavy traffic. Almost feels like it's bucking

New issues

When in park and I put the AC on, RPMs drop to 400 and car shakes.

When applying brakes (only about 10% of the time) the RPMs will drop to 400 and car will shake.

New plugs and wires have been installed along with pcv, O2 sensor, camshaft sensor.

I have zero codes or stored codes. I can't hear any vacuum leaks so I'm going to do a smoke test soon and check my coil pack. Any other suggestions? It's my work vehicle/daily while my show car sits inside all day lol

Thanks!

wmburns · Jun 19, 2017

This is a chicken and egg problem. IE, is the misfire being cause by the low idle or is a misfire causing a low idle.

As always suggest starting every trouble shooting session with a through review of the battery and charging system. Please don't make the mistake to think this couldn't be charging system related (a drop in system voltage can also cause this).

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Here's some more information on how to trouble shoot idle issues.
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

How's the state of repair of the AC system. Does it short cycle? Low on Freon? The PCM maintains two IAC trim values. AC on and AC off. IF the AC is short cycling THEN the PCM will not be able to re-learn new AC on idle trim values. EXPECT poor idle performance with the AC on.

Here's some information on an affordable windows based ODB2 scanner. Using an ODB2 scanner very easy to find out what the IAC duty cycle is when the problem happens. Also easy to check the ODB2 mode 6 misfire data by cylinder. This could help focus the trouble shooting (misfire or IAC).

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

Htxgen · Jun 20, 2017

Did some troubleshooting today. Here's what I found. Again, zero codes, stored codes or pending codes.

PCV grommet was VERY loose. Changed it. Did not change rough idle or the ac issue

Tested battery and alternator - all good

Tested coil pack - all 6 worked and showed spark.

This leaves me to a few final possibilities. Clogged fuel lines (fuel filter has been changed but may need it again since I've ran some cleaner through the tank) and/or alternator might be starting to go bad.

Electrical issues im having - when rolling down window, RPMs drop and car shakes

When braking (10% off the time) RPMs drop and car shakes

When in park and AC is on, RPMs drop and car shakes but eventually comes back to normal range.

I'm running out of ideas. I ran a can of B12 through the gas tank. Every time I add a fuel additive it seems to improve how the car idles. Might be leaky injector or clogged fuel lines?

wmburns · Jun 21, 2017

Htxgen said:
Electrical issues im having - when rolling down window, RPMs drop and car shakes

^^^This makes me wonder if the alternator voltage drop test was actually done. Because the symptom above sounds exactly like a car with an alternator that NOT keeping up and the system voltage is dropping.

I'm having problems understanding why not run such a simple test that doesn't take $$ tools and takes only minutes to run. If I have to convince you that low system voltage can CAUSE a misfire, then I'm not sure I can help you.

If you suspect a fuel injector problem then consider an injector cleaning/flow test service such as InjectorRX.com. Once cleaned and tested they are as good as new. Cheaper than new. The best part is you will get a before and after flow report that will let you know exactly how clogged the injectors were.

For seriously clogged fuel injectors I have doubts that 100's of gallons of anything added to the gas tank can clean them. Further the gas treatments won't work if the fuel injector is defective.

Htxgen said:
PCV grommet was VERY loose. Changed it. Did not change rough idle or the ac issue

Did you also change the PCV valve? After all it is a maintenance item.

Htxgen · Jun 21, 2017

wmburns said:
^^^This makes me wonder if the alternator voltage drop test was actually done. Because the symptom above sounds exactly like a car with an alternator that NOT keeping up and the system voltage is dropping.

I'm having problems understanding why not run such a simple test that doesn't take $$ tools and takes only minutes to run. If I have to convince you that low system voltage can CAUSE a misfire, then I'm not sure I can help you.

If you suspect a fuel injector problem then consider an injector cleaning/flow test service such as InjectorRX.com. Once cleaned and tested they are as good as new. Cheaper than new. The best part is you will get a before and after flow report that will let you know exactly how clogged the injectors were.

For seriously clogged fuel injectors I have doubts that 100's of gallons of anything added to the gas tank can clean them. Further the gas treatments won't work if the fuel injector is defective.

Did you also change the PCV valve? After all it is a maintenance item.

I did test the alternator at advanced auto. It was fine. But it did drop when I turned the AC on. I was always under the impression that alternators fail pretty quick (in my experiences but I'm far from an expert) rather than 12 months or so of this rough idle issue I've been having. But lately I've been thinking it's the alternator because it's showing a lot of symptoms of it failing.

Yes, I changed the PCV valve along with a new grommet. The oem one was so loose that I was able to barely use any force to pull it out. Now it is on there pretty snug and tight.

Htxgen · Jun 21, 2017

InjectorRX is like 15 min from my house also. Probably a good idea to do that. Thanks!

wmburns · Jun 21, 2017

Sooooooooooo you didn't didn't perform the voltage drop test in addition to having the alternator tested.

I have failed.

Neuron · Jun 22, 2017

wmburns said:
Sooooooooooo you didn't didn't perform the voltage drop test in addition to having the alternator tested.

I have failed.

No you are just running up against human nature, hang in there your advice is quite helpful, I enjoy reading your posts and their answers and even learning something about these electronic monsters.

Nater_Potater · Jun 22, 2017

Neuron said:
No you are just running up against human nature, hang in there your advice is quite helpful, I enjoy reading your posts and their answers and even learning something about these electronic monsters.

Ditto!

hotcobra03 · Jun 22, 2017

Have you done a compression test?

What happens if mass air flow sensor is unpluged

hotcobra03 · Jun 22, 2017

Dont rule out clogged exhaust

wmburns · Jun 22, 2017

Htxgen said:
InjectorRX is like 15 min from my house also. Probably a good idea to do that. Thanks!

If you go there in person be warned. They have a different price structure if waiting verses return mail. I was once charged the "wait" price when I dropped off a set and told them to mail them to me. I was somewhat mad to have paid the $$ for quick turn around but still having to wait. It's the girl with the British accent that will do that.

I make darn sure when I drop off a set to be mail back to me that I'm charged the regular price.

Now by all means if it's your intention to have then done while you are waiting then by all means.

Pet the dog for me.

Chasing Misfire With New Symptoms

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