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Discussion Starter #1
'88 Vulcan 6 won't start or run without pumping the gas pedal. Engine dies when pumping is stopped.

Tried thickening the TPS drive tang with layers of Mylar tape which increased the idle TPS voltage 40% (without increasing the throttle angle--more gas without more air, theoretically). No effect. Still had to be pumped.

Verified continuity from TPS plug to 60 pin computer plug.

Would not start with TPS electrically disconnected. Would not start with TPS electrically connected but mechanically disconnected (unmounted). Would not start unmounted with fabricated plate to hold TPS wiper in idle position or wiper held in higher-than-idle position. (Also tried spare TPS.)

Test light connected to one injector plug blinks at same frequency as spark when gas pedal is not moving (cranking with starter). Test light stays on while gas pedal is going down.

Fuel pressure measures 40psi. Removed all injectors and tested them at low pressure (with aftermarket pump designed for carbureted engine): All squirt fine stream.

Bad MAP?

It seems like the injectors have a fixed on-time regardless of throttle position--unless the TPS signal is increasing--and this on-time is too short for the air coming in, even at idle. Or the on-time is not fixed, but always too little for the air (lean). How big of an air leak would there have to be to negate a 40% increase in idle fuel request?

Meanwhile, engine runs fairly smooth while pumping the pedal. Fuel seems to build up (in manifold) sometimes so that engine can run for a couple seconds without pumping. Runs longer if throttle is closed during this time. Holding throttle open during this 2 second run-on makes engine bog (lean out).

Verified spark in all 6. Compression is 150 +/- 2.5 psi in all 6. Valve timing checks out with air pumped into spark hole. (Probably wouldn't get 150 psi if it was wrong anyway.) No evidence of fuel leakage into fuel pressure regulator.

Again: Bad MAP? Air leak? Senile computer? I presume air in the fuel rails would quickly get flushed back to the tank or blown into the cylinders (no need for bleeding)?
 

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Unplug the MAF electrical connector and see how it runs.

Run the COMPLETE code test (KOEO, CM, and KOER) and report back with the results. Getting the codes is ALWAYS the first step when diagnosing any engine related issue.
 

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I only have MAP, intake air temperature, coolant, and throttle position sensors. Both temp sensors test good. I don't know how to test the pressure sensor other than, when the weather was warm, the engine stumbled along long enough for me to disconnect the MAP. The rpm quickly increased until it quit.

I don't have a flow sensor. The TPS does that job.
 

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As for the codes. It's not "throwing" a code and the only codes stored tell me the engine is too cold to get good readings.

21, 22, 24, 52
 

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Sorry, I also have an O2 sensor. (unheated, 2-wire). I believe low or no volts from the O2 make it run rich. I'm lean.

I can make it "run" (by pumping) with the IAC valve either electrically disconnected or removed and holes blocked off.

While pumping the gas it is not in idle mode so the valve can't help.

I cleaned and tested the valve anyway.
 

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I found a pressure/volt table for the MAP sensor, but I really didn't need it. The voltage at the sensor was 2.6 constant. I knew I had a problem when I got such a high reading at zero pressure. I confirmed a 5V supply to it and I went ahead and hooked up my handy refrigerator compressor and pumped it down to -20 mgHg and it stayed at 2.6 all the way.

I tried doing an autopsy, but it is all potted so I just demolished it trying to get it apart. Hopefully this is the only problem.

I ordered a new one from China which will take more than a week. 10 bucks, free shipping--NICE!
 

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Sorry, I also have an O2 sensor. (unheated, 2-wire). I believe low or no volts from the O2 make it run rich. I'm lean.

I can make it "run" (by pumping) with the IAC valve either electrically disconnected or removed and holes blocked off.

While pumping the gas it is not in idle mode so the valve can't help.

I cleaned and tested the valve anyway.

The IAC valve bypasses air around the throttle body butterfly valve to regulate idle speed. An old boss had a 1989 or close Taurus that he drove for a week with using the method you are using. We replaced the IAC valve and it fixed the issue. You can ohm out the coil/linear motor in the IAC that will tell you if it has a blown coil but that is all and won't really tell you if it is not responding right. On older Taurus it seems like the IAV is a total shut off where the newer ones seem to set a fixed idle. I have also seen this happen on a Subaru. This is my experience but maybe it's a MAP but how good is a $10 MAP.



See attached pic.


If it were a MAP why does opening up the throttle plate enables you to run the engine? Is the fixed high pressure keeping the IAC valve shut and it doesn't use engine RPMs feedback to maintain idle. I know it is used to calculate mass air flow using temps and pressure were as a MAF is a calibrated heated wire and the temp change is related to air flow.
 

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Discussion Starter #10
Luckily, I don't have a MAF. MAP controlled engines are tolerant of vacuum leaks.

That $10 MAP is, no doubt, the same one Autozone is selling for $50. Go to the source!
 

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Update:

Although the problem was the MAP sensor, I found that out by accident. The schematic in Chilton's uses a variable resistor symbol for the MAP. It is not! The MAP is a variable pulse width generator. You need your scope to measure the output. I got lucky when I decided it was faulty due to lack of output voltage variation.

I bought a new MAP from Hong Kong (sinyoauto2011) and it was a POS. The thing fired right up, but driveability was crap. It had a 2 second recovery time after any decrease in pressure. i.e. Whenever I lifted my foot, partially or completely, it would bog when I put my foot back in it. When starting from stop it would launch really hard after the delay, spinning wheels etc. Gas mileage was cut in half to 10mpg. This new MAP has no numbers on it so I wonder if it failed final inspection and was rescued from the trash.

I found an old Lariat F150 in a boneyard with the hood off and no engine. The electrical plug was still connected to the MAP, but not the vacuum so rainwater could have entered it, but for $5 it was worth trying. I swapped it out in the parking lot and it has been running beautifully ever since with driveability and gas mileage restored.

I'm trying to get a refund, but even if I do, with gas at 3 dollars per gallon I have paid dearly for this "new" part.
 
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ddan
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