Random Power Drop at Highway Speed with A/C On and Try to Accelerate

Mikey1968

Mikey1968

Member
Nov 1, 2003
232
0
17
Kommiefornia
#1
My stock 89 GT will drop power when I'm cruising at 75 and I punch it while the a/c is on. Here's the effect, when at 75 with a/c on, I will push the accelerator to the floor to pass a truck and nothing happens except my speed slowly drops to 60 and all of a sudden catch. Feels like the throttle pedal just disonnects itself. Doesn't seem to be ignition as I still have fire, isn't sudden, just acts like I'm lifting off the throttle slowly even though I've got the pedal to the floor. Once it catches at 60, I can accelerate like normal. This is random and rarely happens but it sure wakes you up when it does. Only when a/c is on, too.

Never happens during any other acceleration, only at high highway speeds when I push the throttle that last bit to the floor.

I'm thinking MAP sensor intermittent....maybe a spot on TPS? Sticking EGR? :shrug:

Anyone ever have this problem?

Help!?!
 
#5
Steve's idea was my first thought too - make sure the WOT relay is disengaging the AC clutch.

You didnt relate RPM's that I saw, but for me at 75, I'd be lugging and a lack of WOT relay function could make a difference in SOTP feel.

Otherwise, I'd pull codes. In 5th at low RPM's, you can display little issues that normally arent felt - the gearing makes them feel like slow-mo and strains things that normally arent strained.

Good luck.
 
#6
You guys thinkin' that the a/c clutch is engaging at the exact same time I push on the pedal? ....and with the stock gears at high speed....power drops?

I'm wondering why it wouldn't do that all the time and/or when at cruise when the a/c clutch engages?
 
#9
The thing is that it sounds like you are at low RPM and in a tall gear - that's the toughest time to try to make a car accelerate.
Any extra parasitic drain can really strain things.

When you got to WOT (2.71 volts over your baseline TPS setting), the WOT relay (located on the passenger side fender under the MAF) shuts down the compressor to give you all available power.

If this doesnt occur, then the compressor would stay engaged at WOT and arguably hurt performance a bit.

The WOT relay wiring is known to vulcanize and the relay gets baked pretty good itself. At rest, the common and N.C. terminals allow the compressor to engage (the relay needs to be functional to make the compressor stop at WOT).

You should hear the relay and clutch click off. I test this stuff with the car stationary and the hood up - you can whack the throttle open real quick (NO need to hold it open - just stab it and let go) to see what's going on.
Dont over rev the motor or anything - I never rev over 2500 RPM while testing WOT like this.

Good luck.
 
#10
I can check it but why would I actually lose power when I mash the pedal, not just fail to add power? Know what I mean? I guess it could lug and actually slow down rather than speed up when I mash the pedal.....

I'll print out your instructions and give it a try....thanks!

I'ts gonna be hard to reproduce it as it rarely happens...very intermittent.
 
#11
If it's that bad (hard for us to tell from here), I'd simply start with code retrieval. Unless you're going to WOT at or less than 1500 RPM, the car shouldnt slow down.

And codes would be where I'd start.
FWIW - if you kick it down a gear, does the car accelerate around trucks (etc) decently, or is the issue still there?

Good luck.
 
#12
Well, pulled codes but the two codes set were from messing with my fuel pump....I forgot to clear them when I fixed my fuel pump. Rats!

It did it one more time. 2000rpm at 75 with a/c on, floor the throttle and power slowly drops off to 60 and then bangs back to normal acceleration on up to 85...I don't go any faster than that. You can feel the thing catch at 60 .......coast and instant bang to start accelerating nomal again. MAP sensor? EGR popping closed? TPS? Whatever it is it's rare and quick...hence my thinking it's electrical.

What operates at a load at high speed? and ONLY when a/c is on? and REDUCES power?!?
 
#13
OK, now it did it without the a/c on. Was not hot out this time. Seems to only happen going up a hill in 5th gear. I haven't tried downshifting yet. Very random.

I pulled codes and only the usual unrelated 67 was set and the "All Clear" 1s.

I'm beginning to wonder if the new TFI is bad. I'm thinking it's some problem in the WOT mode. That's a pretty simple mode for the EEC-IV....not many sensors used.

Don't know if it matters but I daily drive from to/from 2300 - 4500 feet.

Any ideas?
 
#14
Are you flooring the car in 5th gear and expecting fast acceleration with the AC on or off?

Do not ever floor the car in 5th gear!
5th gear is only for cruising,not for acceleration.

You have to downshift out of 5th gear if you want to accelerate fast:D.
 
#15
I'm not sure you guys read his first post thoroughly.

It sounds to me like he's going 75 mph in 5th, pushes the pedal to the floor, and the car looses ALL POWER and starts COASTING down to 60, at which point it starts accelerating again.

We all know a stock 5.0 doesn't do much in 5th at WOT, but it shouldn't decelerate when you mash the pedal.

The EGR doesn't sound like a likely suspect to me, but you could un-plug it and see what happens.

If your TPS had a bad spot, I would expect power to drop off when you hit that spot, and not come back until you moved the throttle. I wouldn't think it would come back when your speed dropped a certain amount.

See if you can replicate the problem in 4th gear, at the same RPM's you're at in 5th when you have the problem.

Did the problem start only after you replaced the TFI?

'89 is MAF correct?

Jeff
 
#16
Jeff, I just re-read my comments and I think I had the 75 MPH thought in mind. High load and low RPM (1500-1800 RPM or whatever in 5th at WOT).

Definately point out things you think are incorrect or where I misinterpreted what he said - I take humbling just fine and we all just want to help Mikey get it fixed.
 
#17
Jeff's on the money.

I've mashed the pedal in each gear with no repeat of the symptom. Had no problems today. 89 is MAF.

I had many problems occur all at once....when the wife said she wanted to sell it so I "stole" it from her and fixed all sorts of annoying things involving fuel and spark....new distro w/ TFI, new fuel pump, new ERG position sensor and new gas tank...the GT would die in a left hand turn only and recover in a few minutes. That one drove me nuts until a non-Ford guy causally mentioned the fuel baffle in the tank being broken....he nailed it! Cleaned the 10-pins, MAF, EGR, EGR vac control filter, throttle plates, IAC....reset base idle...on and on.

Now, all that's left is this problem. I have the two good books on the EEC-IV and many years of experience with Fords...plus I'm a EE.

I just can't figure why only in 5th gear. TPS would be all gears, as would MAF and the Baro. My gut is telling me EGR just occasionally doesn't seat mechnically as it slams shut on WOT.

Very random occurance and for just a couple of seconds. Very odd feeling when a moron in the lane next to you is annoying you so you push on the pedal to slowly leave him behind and you slow down instead, then it catches. Grrrrr! Minor in the scheme of things but still bites.

I keep figuring it will finally fail often enough I will be able to track it down. Since I can't make it do it when I want, attempted fixes will only drive me insane. It only happens about once in 4-6 hours of driving!

Thanks for your help, guys!
Mikey
 
#18
...another thing I just thought of....I'm usually in cruise control when I mash the pedal....wifey says it's happened to her with cruise off (but only in 5th for her, too). I haven't studied how cruise works into the EEC strategies yet. Wonder if there's any connection there?
 
#19
Mikey1968 said:
...another thing I just thought of....I'm usually in cruise control when I mash the pedal....wifey says it's happened to her with cruise off (but only in 5th for her, too). I haven't studied how cruise works into the EEC strategies yet. Wonder if there's any connection there?
Click to expand...
The cruse control has its own separate computer. The ECC doesn't know or care about cruse control.

The TPS is a possiblity. The element on many aftermarket TPS sensors isn't as good as the OEM ones. The carbon or whatever resistive material they use may wear off and collect in one spot. That will cause strange problems due to the sudden change in resistance. So will the missing material, because it creates a non-linear zone in the TPS travel. Since the TPS operates as a voltage divider, you can imagine what the computer thinks when the voltage ouput changes suddenly.

Some diagrams are available just in case you don't already have them. The site is a great reference for your Mustang questions

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
 
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