All the VSS does on an '87 is control the cruise?
Correct.
What's killing me is that the idling problem is never consistent. Sometimes when I start the car, the RPMS immediately go to around 2,000, stay there for a few seconds, and then quickly fall back down. After the rpms drop back off, the car surges a few times, then levels out. From then on, the car will idle correctly until the engine is turned off.
Hey, ***** ****
That's WHY I got my '86 Stang.
SD cars are notorious for SUPER MEGA CRAPPY IDLE!! Yes, turn it off, turn it back on, and it's idles "differently". Not always better, but
different.
My best friend bought an '86 Stang brand new. He ordered it and I brought him to the dealership to pick it up. It was 100% serviced at the dealership the whole time he had it. Finally, after ~10 years, he couldn't take the POS idle any more. Also, at that age, things were going. And, dealership service isn't cheap. That's when I got the '86 Stang - now
my Stang.
http://www.veryuseful.com/mustang/tech/misc/Mustang1986_with_Mach1_springs/
As for the idle, when my friend had it, the Stang would drive fine, he'd stop at a stop light, and sometimes the engine would go into SURGE UP/DOWN freak out mode! Sometimes, it would die because the surging was so bad. Of course, the clueless service people kept blaming the TPS. WTF? The TPS has "ZERO" to do with how the engine idles at a stop light. The TPS "ratchet value" (lowest value of the TPS per turn-on) is used to determine that the TPS is fully closed. But, the idle speed is based on a simple closed loop of the RPMs. However, as I said, you then also have the injector pulse width changing because as the idle increases/decreases, so does the injector pulse width. And, you have the EEC trying to keep the engine in closed loop with the oxygen sensors. So, the EEC is adjusting the injector pulse width to keep the oxygen sensors happy. That effects idle speed a lot. That then triggers the idle adjust algorithm to adjust the IAB/IAC. Of course, the EEC is adjusting the IAB/IAC as it's also adjusting the injector pulse width to maintain a closed loop. Depending on how "things sync up", a very bad feedback loop can be created and you can get the horrible surging.
Again, also remember that this was all done when computer closed loop control and electronic engine control were pretty new. Then, add in that the engineers had to program in assembler (horrible) and had to cram as much as the could into what was expensive EEPROM back then. Of course, GM was "smarter", and they socketed their EEPROMs. GM knew that once a few million of those cars hit the roads for the first time, there was likely to be problems. GM also took the approach to have a better computer and control. I figure that the GM computer was $7-$20 more (at that time) than the Ford units. So, who was smarter - GM or Ford? You'd really need to know how much each company spent on warranty issues and how it really effected their future business to say for sure. Basically, many engineering decisions are never cut and dry, there are always trade-offs.
After I got the Stang, I changed the "standard" stuff that caused surging idle (EGR and IAB/IAC). But, the idle still sucked and it still surged. But, it surged "less". (That's like saying "she's less ugly"
) The IAC/IAB sticks after ~25K miles because of carbon buildup. So, clean or replace it. Same for the EGR.
The MAF systems idle "better" for a few reasons.
Bottom line is that ALL 5.0 HO engines
suck at idle! The SD engines suck more and are very prone to "random" surging. As I said, from what I remember, that was do to the simple idle algorithm that was used and crammed into the small PROM. I also looked at the SD idle algorithm many many years ago (over 10 years ago). It sucked worse.
So, going to a MAF-based setup may help with idle. As long as you use a 100% stock MAF and MAF intake setup, the idle may suck, but hopefully, you won't have the infamous "random" bad surging that happens with SD setups.
Of course,
every engine is different.
.... specially for the guys at Manufacturing.
