Need help diagnosing a miss, and one other odd issue.

[Fett]

I will try to make this as short as possible....

Since I first built my engine, I had an odd "miss" while cruising around 2300 rpms....between 2300 and 3000. At just enough throttle to cruise, slightly speeding up....I would get this odd, slight miss. At the time I started shifting around 2000rpms and never felt the problem since.

Last week it started doing it randomly at any RPM. It won't do it if I am 1/2 throttle or more. It will just have this miss while driving...it's hard to explain. The best I can decribe it is...you know when you are in too high of a gear just rolling...and the car starts bucking? It's kind of like that, just not to that extreme. Anyway...

I am not 100% the original "miss" and the new "miss" are related. I started shifting at 2000 about 6 months ago. So it is possible the problem has always existed, and is just getting worse now. Or it could be two different issues.

Since then, I have changed coil, added MSD, dizzy, and cap. The only spark items I have not changed are the plugs and wires.

Fuel is set at 40 with the vac unplugged, 15* timing, and 24lb injectors/MAFS.

In the past week I pulled each plug, checked them and the gap. They all looked good. I did a compression test, which passed.

Today I tried to use a Star Tester and an NGS to do a cyl stepdown test. With the KOER it would not communicate with the PCM. With the KOEO, it would pull code 67, which is "car in gear, or AC on". Neither of which are true. It has always been a 5speed, so I don't think it is an issue of a closed NSS circuit.

The other issue is everytime I start the car, the idle rises and falls between 500rpms and 1000rpms....it will do it until I quickly stomp on the gas, which quickly brings the car up to 1500rpms...then it will idle fine. It does this EVERY TIME I start the car. I checked all my grounds, and they all seem to be good. I cleaned the surface of each one of them off to be sure. The idle issue also happens when my fan kicks on...and again can be fixed with a quick rev.

Now I am wondering if the PCM itself is bad, because it won't communicate with the star tester with the engine running...and maybe it is a ground issue with the PCM itself. OR....it is not communicating with the PCM because it thinks the car is in gear.

The PCM is out of a 91 GT, 5speed.

Any thoughts would help, I am next going to clean my injectors and maybe replace plugs and wires. I am trying to avoid just throwing parts at my car, I would rather actually find the problem and solve it.

Thanks.

 

[superhuaman]

sounds like it could be an iac issue

 

[Fett]

sounds like it could be an iac issue

For the idle issue, not the miss, I assume?

It could be the IAC, it is new....but that doesn't mean it is good. For craps and giggles I unplugged it while the engine was running, and the idle didn't stumble as much as I would think it should have. So I removed it and cleaned it out...same difference. I might replace it anyway, once I figure out the miss problem.

 

[HISSIN50]

Code 67 is keeping you from accessing KOER. Make sure that the neutral switch is functional since the car was in neutral and the HVAC was off.

Chris, can you see PIDs? I'd have an assistant watch IAC and EGR function (those were the first two to come to mind) while the miss occurs.

Good luck bud.

 

[1990Fox]

Since then, I have changed coil, added MSD, dizzy, and cap. The only spark items I have not changed are the plugs and wires.

Thats where i would start.

 

[Fett]

Thats where i would start.

Yea, but I don't want to throw parts at the car in hopes that something works. These aren't "old" plugs and wires....granted, new doesn't mean old...but this engine has only been together for about a year and only has roughly 8k on the clock. I might end up going that road, but I would rather find out what the problem actually is...THEN fix it.

 

[1990Fox]

codes? atleast they can tell what cylinder is missing.

 

[Fett]

Code 67 is keeping you from accessing KOER. Make sure that the neutral switch is functional since the car was in neutral and the HVAC was off.

Chris, can you see PIDs? I'd have an assistant watch IAC and EGR function (those were the first two to come to mind) while the miss occurs.

Good luck bud.

All I have is an NGS and a Star tester...which is great at pulling codes and running random tests, but they don't monitor PIDs....as far as I know. I wish I could, that would save me a ton of time.

I know the EGR is crap...even though I am not getting any EGR codes anymore. I was getting 34, then I cleaned the crap out of the EGR and reinstalled...but it doesn't seem to be kicking up at all...but again, I didn't pull any EGR codes this time. Just 67, and 10.

I wonder why I am getting 67...I supposed the NSS could be bad.

Do you really think EGR or IAC could cause a miss like this? Like I said, the IAC is new and looked very clean...but I don't think it is working properly. Isn't it the job of the IAC to kick up the throttle when the engine goes under a load? When the AC comes on, or say when the fan kicks in, the IAC is supposed to even out the idle? If the IAC can cause a miss like this, I might lean towards that...because I could see where it is the reason behind my engine idle issue.

 

[LtDavis]

hmmm sounds a lot like what my car does. I can hear what seems like a slight miss at idle and sometimes it does it while driving, also causing the car to buck a tiny bit once in a great while. I can also hear what sounds like a lifter or rocker at the back of the passenger side valve cover going bad, I was told that can cause a valve to stick which could be causing the problem. I'm at 12 or 13 degrees timing for what its worth.

It seems like everything started going bad on my car when I installed the off road h-pipe...

 

[HISSIN50]

You got it Chris - the IAC is really just for idle compensation. But it also has ancillary participation for things like dashpot function.

But if the IAC is going haywire (as seen by IAC duty cycle bouncing around when the miss occurs, which could be brought on by an EGR issue. The EGR wipes out performance, so to maintain speed, the throttle is open a little further, which makes the engine more efficient) it could have been worth lookin at. I was grasping at straws, and if you could see PID's, it'd be two things to waste 30 seconds checking. I wouldnt bother really otherwise.

The plug wires idea isnt a bad one if you happen to have a spare set of wires around. It sounds like no though.

I'm not as conversant with MAF's as the other guys but does anyone think the MAF can cause this issue? I'd certainly clean it with electrical contact cleaner or MAF cleaner if you havent (the 94-95 MAFs are real finicky so I do this often).

Those are my wild guess/random thoughts Chris. Sorry to not be of more help.

Lt. Davis: I've had luck with fixing a light occasional buck by cleaning the PIP sensor off. I didnt suggest this for Chris since he has a new dizzy. Once again, it takes 30 seconds so it's worth a shot.

Good luck.

 

[Fett]

Yea, the MAFS has been suggested. I did clean it out well, but it is a ProM...and I know those tend to suck. So it could still be that.

That is where I am at now, I have a bunch of ideas, and I keep trying to eliminate them one by one...I just want to do it without throwing a bunch of money at it, hoping one of the parts I replace will fix it.

I am going to try a known-good IAC tomorrow, just to see if it makes a difference....then maybe my friends MAFS.

I figure I can get 1 #8 wire and try it on each cylinder to see if the problem goes away.

I have until the 24th to figure this out...I WANT 12s that day. But I won't bother trying if I am still having this miss issue.

 

[jrichker]

Do you have a CAI or stock airbox? If a CAI, try clocking the MAF. Turbulence
through the MAF will produce the type of symptoms you are describing.

Help for the code 67 - the Neutral Gear Switch is in the upper RH corner of the
diagram.

Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

 

[Zero_chance]

You dont know how many times I wish I could use the WDS or IDS at my shop to look at PIDS. NGS is only good for pulling codes quickly unfortunately and a few other tests that have limited uses. Itd be nice to hook up to my car and be able to lok at the power balance display.....

 

[LtDavis]

Hey Hissin, whats the PIP sensor and where is it located (somewhere on the distributor I assume) I'm all for a 30 second fix!

 

[superhuaman]

pip is that black thing down inside the distributor

 

[Fett]

Ok, we might have figured it out....hopefully.

Today I switched out my MAFS with a known-good one, and it was still bucking. We still believe it is running lean...I have my pressure set at 52psi with the vac line off, and I still don't have any signs of a car that is running rich...plus my plugs were clean as if the car was running lean originally, when the fuel pressure was set at 40psi. Of course that is pressure and not volume.

Anyway, two weeks ago I found that I had an EGR code. So I found a used EGR, cleaned it out with carb cleaner and installed it. Two days ago I didn't have an EGR code, so I assumed that wasn't the problem. But after driving the car we found that the bucking only happens while cruising. So we unplugged the vac line to the EGR, and capped the line off. The EGR is still plugged in and still on the car, but there is no vac going to it. I test drove it and the bucking seemed to go away.

Now I don't know if I should leave it how it is, by leaving the EGR on the car and plugged in...just leaving the vac line off. Or if I should buy one of those EGR delete plugs off of eBay. I am not going to replace it, as I have no need for it....plus I don't want to spend the money replacing a part that I really don't need in the first place.

Now then, can a faulty EGR cause a lean condition? Or are these two different problems?

Oh, and has anybody used an adjustable EGR spacer to help with idle conditions? When my e-fan comes on at idle, the car starts surging horribly, until I give it a quick rev.

 

[superhuaman]

get a plate if you dont have emissions and you dont need it to prevent detonation.

now, there are two types of faulty egr valves:
stuck closed-causes pinging
stuck open-causes stumbling and rough idle (rich since egr displaces air)

and i believe you are referring to an adjustable IAC spacer not an EGR spacer.

 

[Fett]

Yea, adjustable IAC spacer, that's what I meant to say.

I actually believe my EGR was not stuck either way. I believe it was opening up too much at key times. I believe my original one was stuck closed, but this one opens at cruising....as it should, but maybe it is opening up too much. Either way, the car seems better with the vac line disconnected. If it is the EGR valve causing the problem, I am just going to remove it. It is just an emissions item anyway and I have no use for it.

I just don't know which way to go about removing it. I can just leave the vac line unplugged, use a blockoff plate and an EGR delete plug in, or get a custom chip and have the EGR removed that way...which would also allow me to remove pretty much EGR sensor and the other two vac sensors.

 

[superhuaman]

the best way is to use a custom tune, obviously. the egr delete plug in is just to keep the check engine light out. but the computer thinks the egr is still installed and working, so its not gonna back off timing, and pinging may result.

 

[Fett]

Ok, if I were to just leave the vac line unplugged, the PCM would know that the EGR is not opening. Would that force the PCM to retard the timing? If so, under what conditions?