Engine gives gas when I brake (FBird `91 Aut.)

Could anybody help me? Shortly after starting the following happens when I brake moderately: The engine gives gas while the car decellerates. The tach
falls to 500/min and then it jumps to more than 1000, then it falls to 500 and so on. While standing idle is constant at 700/ at full A/C: 800.
When I interrupt breaking, the car runs 100s of yards in this way (@25mph). Sometimes the engine stops running while braking(tach falls to less than 500, engine shivers, then it stops).
This mysterious thing has begun after a battery change (for the necessary reprogramming I did drive 40 min on a highway). Repeating the reprogramming did not solve the prob. Generator is new.
Who knows the prob? What gives the signal that idle is too low? Please answer here.
Excuse my English; I am from Europe.
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Georg,
I purchased my Firebird ('91 Formula) in 1988 with this problem. It had been initially, and incorrectly diagnosed as being a fault of the throttle position sensor (TPS). Upon further diagnosis, the fault was actually with the injectors themselves. It seems that the injectors were intermittently sticking; most noticeable during throttle transitions and at small throttle openings.
You did not say which engine your Firebird has, but if it is a TPI V8 (5.0L or 5.7L) it will have 8 separate injectors (1 for each cylinder). I am not sure of which fuel injection system is used on the 6 cylinder models. Either way, a technician with an engine analysis tool should be able to diagnose if the injectors are the problem quite quickly. The injectors provided on the TPI engine Firebirds and Camaros are cleanable at a shop with the injector cleaning equipment, but it is not recommended to abrasive blast clean them as the plating may (read will) be removed during the process. After which time, the injectors themselves will rust internally, making your problem even worse. If cleaning the injectors does not solve your problem, you will need to replace them - be warned the stock GM injectors are very expensive. Also note that most shops charge for about 6 hours labour to disassemble, and reassemble the TPI plenum and fuel rails.
Franki
Georg Hanuschik wrote:

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All 1991 F cars use the GM/Rochester Multec multiport (one per cylinder) design.
There are only four engine choices, a 3.1 vin code T, a 5.0 vin code F TPI engine, and a 5.0 vin E TBI motor, and a 5.7 vin 8 MFI motor. Joe--ASE Certified Parts Specialist & 10th Ann.Club Tech Director '80 Carousel Red Turbo T/A, 27k orig. '79 "Y89" 400/4 speed 10th Ann. T/A, 57k orig '84 Olds 88 Royale Bgm 2 dr, 307 "Rocket" (lol), 143k and still going.... '91 S10 Blazer 4.3Z
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How'd you get a 1991 model in 1988?
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Bruce Chang wrote:

I worked hard at it - but it was just after I finished my typing class :-(
Franki
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reprogramming
Pull the fuse for the 'ECM' for 1 minute. Which should clear any trouble codes, and programming that the car can 'learn'. Then drive it, and check for trouble codes.
If no codes show up, you should get the code for 'ECM' ok. If you do not get this code, you have a ECM problem. Which may be as simple as a broken wire or bad contact.
From this side of the pond it almost sounds as if the cruise control is trying to activate when you are braking, then is leaking vacuum when you are accelerating. Charles
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