RE: IML: 81 Imperial changeover

The fuel pressure switch is there to force the control fuel pump to run at full speed for a second or two when you first turn the key to “ON”, in order to purge any vapor that has formed in the plumbing. It shorts a wire to ground momentarily, like an oil pressure idiot light, which overrides the computer signal which normally would be in control of the amount of fuel delivered to the nozzles. This is why, when the FPS fails in the normal way it fails (open), the car is very hard to start, because there is no purge cycle, thus the car is starved for gasoline in the first few seconds of cranking. A normally functioning FPS will release the wire to float as soon as the pressure in the plumbing comes up to 20 PSI or so.

In your case, however, since we suspect that the control fuel pump is already putting out too much gas, I doubt that disconnecting the fuel pressure switch will make any difference at all. As I recall, you already measured the resistance of the FPS, and found that it was, if anything, a little too high – which would cause the opposite problem from the one you are having. In other words, right now, I don’t think your FPS is doing anything at all for you.

Just in case I am guessing wrong, you can always fool the system into thinking the FPS is operating normally, by momentarily grounding the wire that is normally connected to it. This takes two people to do, however, since it must be grounded BEFORE you turn the key “ON” and then taken off ground immediately after the key is turned on. The consequence of not taking the wire off ground would be to flood the engine with gas until you take it off. If you read again my last paragraph in the previous message, maybe this will become clearer.

I’ll be away from now until Monday, so don’t worry if I don’t respond for a few days.

Dick Benjamin

From: mailing-list-owner@xxxxxxxxxxxxxxxx [mailto:mailing-list-owner@xxxxxxxxxxxxxxxx] On Behalf Of arbie104@xxxxxxxxxxx
Sent: Thursday, February 23, 2006 5:00 PM
To: mailing-list@xxxxxxxxxxxxxxxx
Subject: RE: IML: 81 Imperial changeover

Will the car start if I just unplug the FPS? What is the FPS function? I probably won't have time to try this till Saturday. Thanks

-------------- Original message --------------
From: "Dick Benjamin" <dickb@xxxxxxxxx>

Yes, you can check the pressure that way. The pressure there can get up to 80 PSI, so be careful to have a secure connection to your gauge.

The two wire connector on the pump is the one to check. One wire is ground (I think it is black), the other one (I can’t tell you the color because I am colorblind), is the one we care about – it varies between almost Zero to almost 12 volts, depending on the fuel demand signal put out by the computer, which knows the throttle position, the fuel flow rate, the RPM, and a bunch or other things, like temperature of the air, fuel and coolant, exhaust oxygen, altitude etc..

The power module basically is just a DC coupled power amplifier, which takes the computer output and boosts it up to a strong enough signal to operate the pump motor. One of the failure modes (usually caused by someone shorting the output to ground) is a failed transistor in the final output stage. Depending on which one failed, the power module might be putting out a solid 12 volts all the time; this would also explain your symptom.

The first thing I’d like to have you do is to pull the wire off the FPS as soon as the engine starts – to see if that reduces the fuel flow to something reasonable and the idle becomes normal. If this works, forget all the other fiddling around – you’ve found your problem!

Dick Benjamin