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Discussion Starter #1
Hey Guys,

I have an '81 Turbo that I've swapped the head and manifold on. After the swap, the engine has developed a strange power drop that I need help in diagnosing. (I don't remember the problem being there before)

The engine makes great power from idle all the way to 3500 RPM, but falls flat from about 3500 to 4200 RPM. From there, it starts making full power again. I would guess the drop to be about 25%. Even when the power drops off, the engine sounds fine...no missing, backfires, etc.

Apart from this "notch", the engine runs great. It always cranks and idles smoothly, has good throttle response, turbo builds full stock boost smoothly, and never pings or backfires through the intake...nothing at all to indicate mechanical problems, bad timing, or bad fuel mixture.

Sometimes I can seem to clear up the problem by applying full throttle on and off a couple of times, or running the car at about 4000 RPM for awhile (like at highway speed). It's most obvious when I lock the transmission in 1st or 2nd gear (at lower rpms and mph) and open throttle. Car pulls great until about 3500 rpm, power drops off abruptly, rpms build (slower) to about 4200, then power comes back in as abruptly as it dropped out. I'm baffled.

I thought:
1: I've got the valve timing off one tooth. But why would there be a "notch" in powerband? Wouldn't this bias the power to either the low or top end?
2: The ignition timing is off. I don't have a light to strobe it, but I didn't change the timing from where it was before. (though I did unbolt the pickup during the work) Again, if the ignition timing was off, wouldn't this bias the power to either the low or top end?
3: Restrictive exhaust. Well, the Cat Converter is hollowed out, and a new muffler is installed. Again, wouldn't this just hinder top-end power?

Has anyone had issues like this before? I need suggestions on what to look at/how to fix. Also, if anyone knows of a good Zcar shop/specialist in the Louisville KY. area, please let me know.

Thanks
ZcarMan
'81 Turbo
'83 Turbo 5-speed
Louisville
 

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Here's my thought: I'm guessing that the timing is as you said way off and causin gthe car to want to detonate. The knock sensor is pulling timing way out and causing the loss of power. As the timing is pulled out the car responds and picks up agian. Your going to have to get a light and check it. Try to retard the timing a little and see if this helps any.

Cody 82 ZXT
 

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I like this possibility, as my '81 Turbo, which I finally just got running well yesterday, does seem to do this as well. If this diagnosis is correct, then perhaps some higher octane gas should be tried. I have 91 octane in mine (first tank since major rest period for the car). Maybe I will try a tank of 93 and see if it makes any difference. Manual does say that 91 is needed.
 

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Hey Z-Car man, my car does kinda the same thing. There are 2 things i was thinking about, Valves like you are. But i just had my timing set correctly and it still does it, so that isn't it for me


The other thing is the TPS on mine, i know mine is bad...just waiting to get a new one and see if it helps.

P.S. if it turns out the be the valves....write a mail


[email protected]


Please thanks

Matt
 

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Shouldn't be the tps - on the turbo, it is only used for idle contact/enrichment purposes.

One thought that comes to mind is that under a certain rpm/load the ecu is running in closed loop mode. Ouside of this range it runs in open loop mode based on parameters programmed into the ecu.

In closed loop mode, if the O2 sensor is bad, you could be too rich/lean.

Valve/timing problems would show up at lower rpms and appear slightly better the higher the rpms, so I don't think that is your problem.

What happens on your boost guage when accelerating through this rpm range?

Are you running stock boost?
 

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Ok so tell me this....if i unplug the O2 sensor while the car is running should i notice a difference in idle or anything like that???


The boost guage goes as it normally does/did before all this started happening.

Yes i have a 5 spd

The RPM's while floored wont go above 3500

If i slowly accelerate the rpms up...i can get them to 5000

Do i have a bad computer??? what computer is compatable with the one in my car so i can run down to the J-yard and pick one up that seems to work better??

Thanks

Matt
 

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Discussion Starter #7
Re: Brad-Man, answer to question

Brad-Man,

The boost guage builds up smoothly to "7" (stock) and stays there without movement, until I back off the throttle. It doesn't move during the power drop.

Thanks for the input.
 
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