Here's What Happened When I Tried to Diagnose My Car Professionally

"Urambo Tauro" (urambotauro)
07/13/2018 at 23:30 • Filed to: diagnosis, wrenching, mustang

One one hand, an engine just needs sufficient air, fuel, and spark to run. But on the other hand, a modern car has so many engine management bits doing so many things at once, it’s easy to get lost trying to diagnose a problem. That’s where !!!error: Indecipherable SUB-paragraph formatting!!! comes in handy...

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Of course, I’m not gonna read the whole thing. Despite this book being specific to 1995 model year Fords, it still covers a variety of engines, well beyond just the ones that were available on Mustangs two decades ago. So there’s some navigating to do.

The journey begins in section 2A: Diagnostic Routines, where I had to choose my own adventure by selecting a specific symptom.

My Mustang was trying to !!!error: Indecipherable SUB-paragraph formatting!!! out when I’d lift off the accelerator (as one does when approaching a stop sign), but there were multiple diagnostic charts that seemed to fit that condition. Should I follow the one about stalling at idle, or stalling on deceleration?

It had been a while since I drove it, waiting to get my hands on a book like this. In the meantime, I had noticed a little corrosion starting on the positive terminal. So I removed the battery, neutralized the corrosion and cleaned it all back up before reinstalling. Of course, this erased all of the codes that I had stored.

So I went for a drive. This would get the engine up to temperature, and bring back any codes (if there was still anything going on to trigger them). But the car drove without any problems. Even the stalling had gone away.

But I didn’t fix anything! The battery terminals were still tight when I went to remove them, and the corrosion was nowhere near advanced enough to start causing issues yet. So the next day, I took it for another drive, and the stalling returned. I limped it back home, and pulled codes- nothing. No stored codes, except for a 1-1-1 “pass” code.

But it was idling roughly. So I chose a different flowchart- chart #6, the “runs rough - @ idle” one.

I dutifully went through my preliminary checks, suspecting that most (if not all) of them would be a waste of time. Sure enough, no easy fixes yet.

“Does the engine idle rough?” You bet it does! “Any codes?” Nope, nothing to see here. Next!

Onward to section 6A, a massive stack of pages dedicated to EEC-IV pinpoint tests. Subsection “S” was found on the 437th page of section 6A.

Unplugging the IAC had no effect, so it was time to turn to subsection KE.

Ah- a resistance check! Sweet, let’s see what we got here.

Hooked up my multimeter, but no matter how much I wiggled the connections, it would not settle on a number. It seemed to bounce mostly around the neighborhood of 170k. Well, there it is. Bad IAC, simple as that.

The IAC was actually my first guess when the problem came up a few weeks ago. As I struggled to keep the engine running on my way home that day, I stopped by the parts store for an IAC gasket. I’d gotten good results out of cleaning it a few years ago, but cleaning it this time around would have been a waste of time. I guess I was fortunate that the parts store didn’t have the gasket.

Not only that, but it gave me a minute to pause and remember to check for codes first. That put me into a stubborn no guesses, diagnose it PROPERLY sort of mood. And here I am with a big fat book that pointed me to the problem within minutes. And it has more extra information than I can shake a stick at!

So anyway, I ordered up a new IAC, and wouldn’t ya know the old one came off cleanly without disturbing its gasket. Go figure. Well, I guess I’ll save the new gasket just in case I ever need it and don’t want to wait for a parts store to get one in.

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But before installing the new one, why not test the solenoid resistance?

Sweet. A nice steady reading, too. Perfect!

Got it installed, put everything else back together, and went for a drive. I’ve already taken it through a couple of drive cycles, and so far so good! That must have been it. (And none of those previous codes have come back yet, either.)

Feels so good to not have wasted money on what I was starting to think (before I got this book) was going to be a bad PCM. Feels even better to be driving it again! Now I can park the truck and take care of some things on that one that I’ve been putting off. A couple of new switches (already diagnosed as bad), and I’m thinking motor mounts, too...

DISCUSSION (17)

functionoverfashion > Urambo Tauro
07/13/2018 at 21:52

YAH, well done! I have a lot of experience with that engine and its diagnostic procedures, but in boats. It’s kind of the standard ski boat engine from 1995-2002, especially in Nautiques. I also had one for almost a decade, transplanted into my old Supra.

Cheers to a successful diagnosis, and NOT throwing parts at the problem!

LOREM IPSUM > Urambo Tauro
07/13/2018 at 22:01

I know an older guy with an old C rown Vic who has been troubleshooting a similar issue for over a year. First time he mentioned it to me my immediate response was IACV, but he ended up down the rabbit hole online reading about people adding maf’s, apparently wiring them to the existing pcm or some such, and pretty much got lost in his own head space. Then he stuck it in storage for the winter and went out of province for a bit . Strangely, I don’t think it’s given him any trouble since he’s gotten back and pulled it back out though.

Maybe I'll buy the guy a new iacv for his birthday or Christmas or something.

Urambo Tauro > LOREM IPSUM
07/13/2018 at 22:10

O r a multimeter ;)

NKato > Urambo Tauro
07/13/2018 at 22:17

Which Mustang is your ride, again?

Urambo Tauro > NKato
07/13/2018 at 22:24

It’s a red 1995 GT coupe. Mostly stock, with just a few tasteful mods. I’ve shared a few wrenching posts, but I don’t I ever properly introduced i t t o you guys . I should do that.

NKato > Urambo Tauro
07/13/2018 at 22:43

In other words, the kind of Mustang that my Crown Vic PI eats for breakfast.

Urambo Tauro > NKato
07/13/2018 at 22:50

Probably lol. None of the mods have anything to do with the engine, and I’m still running the stock 2.73 rear gear .

NKato > Urambo Tauro
07/13/2018 at 23:06

The Killer Queen is also bone stock, but she has some equipment that will turn your day into a nightmare if she feels like it. (Pushbars/corner guards)

Eventually, she will be getting an upgrade, new Gen 3 Coyote engine with 10-speed automatic (and SelectShift functionality, hopefully), good for 400+ horsepower to the wheels.

Gonna autocross that monster.

His Stigness > Urambo Tauro
07/13/2018 at 23:37

As I was reading this I was thinking in my head, “bad IAC motor.” It’s nice I was right.

That book looks to be more help than what technicians get in the online manuals. And the worst part is we usually need the specs for older stuff.

Urambo Tauro > His Stigness
07/13/2018 at 23:54

Yeah, f unny thing is- it was the codes mentioned i n my earlier post that planted doubts in my mind about the IAC being the culprit. Really threw me for a loop, hence the book purchase. And I was hesitant to erase those codes because I’d heard that codes are supposed to erase themselves once a problem is resolved. Is that not actually the case, or does that rule only apply to certain systems (like OBDII, perhaps)?

Tristan > Urambo Tauro
07/13/2018 at 23:59

Yes. I’d love to see more of it! I've been eyeballing SN-95 5.0s on Craigslist a lot lately...

His Stigness > Urambo Tauro
07/14/2018 at 00:12

You know, I’ve never actually scanned an OBDI car for codes. The OBDI cars I’ve always worked on have never had the light illuminated.

But, if OBDI is like OBDII in terms of KAM (keep alive memory), then yes, it will eventually clear itself. With OBDII the MIL turns off after a certain amount of drive cycles completed with no faults. I’m not sure how many that is. But I do know that it takes fourty warm-up cycles for a code to be erased in the computer memory. So let’s say after ten cycles I scanned a car that had a fault but it wasn’t cleared, it will be stored as a history code.

DC3 LS, Fuck Hyundai, now and forever > Urambo Tauro
07/14/2018 at 00:28

IME pre OBD-II, OBD is generally as useful as a magic 8 ball with random parts written on it. The closest it ever came to helping me diagnose anything, I figured out what it was without getting to pull the code. Because the CEL would come on when I was on the highway, but go away once I got off.

I still don’t even know what the thing did. Since it only happened cruising on the highway, I assumed it was an emissions thing. Did a visual check and found some tiny filter cap thingy on a line coming from the charcoal filter had a gash in it. Replaced it and the CEL never came back.

Echo51 > Urambo Tauro
07/14/2018 at 03:32

Huh yeah, the american fords used the EEC-IV stuff too like my escort... IAC is a general classic, doesn’t matter if it’s on a Sierra or an Escort. If ya idle sucks as the only thing, IAC checkup time.

I have to admit having access to seeing signals graphed via OBD-2 on mom’s Focus has spoiled me a bit however. Blinking out fault codes on the EEC-IV isn’t that intuitive imo.

Captain of the Enterprise > Urambo Tauro
07/14/2018 at 07:28

I needed this so bad back when my dad had his 95 F150 5.8. It would stall out and then not start or not start and we were chasing the problem but ended up just selling it.

Urambo Tauro > Captain of the Enterprise
07/14/2018 at 10:38

That’s frustrating. Reminds me of an old (carb’d ) Delta 88 that I used to have, but I got rid of it before I ever found out why it was dieseling.

MM54 > Urambo Tauro
07/15/2018 at 14:43

Glad that the symptoms of a bad IACV turned out to be a bad IACV :)

When the one on my p71 started acting up, it idled high (not low) and made a sound like a goose playing a french horn back out through the intake. It was neat. No codes, just “huuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu”