Hi, maybe I'll try this section...
I have a stock 91 GTA Transam 305 and here is are the symptoms. The engine starts, the whole motor shakes but stays on, I apply some gas, rpm goes up to around 2000 rpm and drops. If I release the gas the engine stays on, If I keep the throttle open rpm keeps on dropping, I hear a couple of backfire out of the plenum and it dies.

So far I replaced all plugs, plug wires, rotor cap, o2 sensor, 2 injectors, adjusted the tps to 0.54 at idle (smooth all the way through), cleaned IAC passage and sensor, adjusted timing, banged my head on the plenum, fuel pressure is at 37 psi, changed fuel filter.

Any suggestions or input on what to do next would be appreciated. Tired of driving the wife's sunfire <g>

 

Sounds like you've got 2 of the 3 basics well covered: fuel, spark, compression...

Time for a compression check.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Try testing the MAP sensor, then check the backpressure in the exhaust system for a restricted exhaust. You can also check for exhuast restriction via vacuum on the intake at idle (should be around 18-20 inches) then open the throttle (in park/nuetral) to 3000 or so, and compare the vacuum reading once the RPM have stabalized, if the vacuum is lower then initial or dropping, the exhaust is restricted.

Back pressure can be checked at the AIR tube connection or through the O2 sensor hole, should be no more than 2.5 psi (GM spec) at 3000 RPMs.

I forget which way the MAP operates, but you should see a sweep from zero to 5 volts or vice-versa when going from no vacuum to 30in/hg. It shoudl also hold the vacuum for at least 30 seconds with no appreciable drop. A MightyVac comes in handy for this.

If you still have stock exhaust manifolds, I'd check everything I could before doing a compression check or leakdown check, it's just a PIA to remove the plugs and install the fitting and reinstall the plugs, 8 times with stock manifolds

[This message has been edited by 8Mike9 (edited June 26, 2001).]

 

fuel, spark and compression is 2 out of three? sounds like 3 out of 4 to me, lol.

 

Hi 8Mike9, I tested the intake vac as you suggested. Not sure if I did it right. I plugged my vaccum tester on an intake in the plenum (the one right next to the MAP plug).

At Idle I get 15 Inch of vaccum. When accelerating, the vaccum goes down and tries to jump right back to 15 inch. I can't get it to 3000 RPM stable, it dies before (vaccum goes down to zero when it does that), but as I am accelerating the vaccum does go down. What does that mean and what's the next step?

Thanks

 

Flesh,

It sounds like Mike is all over this one. Your symptoms are that of a classic exhaust restriction. Check for either a plugged cat or collapsed pipe or muffler.




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Later,
Vader
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"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

Tom:

3 basic: fuel, spark, compression. He had checked fuel (up to a point) and spark. Time to check the 3rd, which is compression.

In any case, "engine shakes" is not part of a stopped up exhaust system. That indicates one or more cylinders are not firing correctly. Your exhaust may be restricted, but that affects all cylinders equally, and so will not by itself cause it to run rough.

Your description of the behavior of the vacuum is not unusual. When the engine is under little or no load, the vacuum is very high, because the engine is pumping air molecules out of the intake tract faster than the throttles are letting them in; under heavy load (think of a stick-shift car in high gear puling a trailer up a hill at low speed with the throttles wide open) the throttles are opened wide enough that air is rushing in to replace nearly all of what the engine is pumping out of the intake, so there is near atmospheric pressure in the intake (low vacuum). Your vacuum is just like that, except it's lower than it should be, which is consistent with an engine that's struggling to stay running, because then you're having to open the throttles more than you should, to keep it from dying.

Next step is a compression test. Although, it's still not impossible that you have one or more injectors that aren't delivering fuel for whatever reason.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Hi, I completely disconnected the exhaust from the engine, same exact symptoms and same exact result to the vaccum test. Sounds good though <g>..

Now what? I'm getting slightly frustrated, but what the hey, it's a nice day out.

Oh and just for the heck of it I disconnected the ECM, runs the same.

HELP.

 

Here's an update.

I did a compression test:
All cylinders are at 150, except #2 is at 155, #6 is at 140.

I added some engine oil to #6, same result, still at 140.

Is that normal?

What's next, come on people, need the car for tuesday.

Thanks.