1998 Ford Crown Victoria

Problem: Runs rough, reduced power, surges and hesitates. Very noticeable when accelerating at partial throttle…but if you open the throttle wide open, it clears up and runs like a scalded dog. Sometimes it idles smoothly, other times it idles awful.

BACKGROUND: The car looks like it has been worked on by every mechanic in town. There is not a wire that has not been unplugged, back-probed, spliced, etc. It is evident that most every sensor has been taken off, put back on, or replaced. Whatever the problem turns out to be, it looks like it has fooled other mechanics in the past.

PARTS REPLACED: It has a new fuel pressure regulator, spark plugs, oxygen sensor, MAF sensor (replaced with a known good used part), and the coils seem to be working fine and the coil boots look good. The fuel filter looks rather new. It looks like the gas tank’s been off it, so I’d say the fuel pump has been replaced at some point.

DIAGNOSIS: When I bought the car, it was having the same symptoms as it does now plus it was running very rich. Fuel pressure was running at 95 PSI, so I replaced the fuel pressure regulator and it’s now running within range (30 to 40 PSI).

The car had OBD codes showing the EGR valve was rich, and the passenger side cylinders running rich, and the drivers side cylinders running lean. There was no gasket on the EGR valve, so I took it off and cleaned the valve (it’s moving like it should now) and replaced the gasket.

All the fuel injectors are clicking along like they should. Like most 4.6 Ford engine injectors, the plugs don’t clip very well but (otherwise) they look good.

There are not (or at least there weren’t a day or two ago) any OBD codes showing up any more on a quick scan. It seems like replacing the fuel pressure regulator cleared up the old codes.

I have checked the TPS with a voltmeter and found the 5 volt reference signal present. I measured the signal coming from the TPS and it varies smoothly as the throttle is opened and closed. However, a mechanic told me that the signal the computer shows for the TPS stays low until the throttle is wide open. This led me to believe that there is an issue somewhere between the TPS and the PCM (computer). I have read that Ford has had some problems with the ground circuit on these cars. The ground for the sensors like the TPS, MAF, etc. are supplied by the PCM. I cut the ground wire on the TPS and added an extra connection to the body ground….but it did not make any difference. I also disconnected the engine wiring harness plug and the PCM plug and sprayed the connecters out with contact cleaner and made sure they were fitting tightly. They seem to be in good shape. The wires from the TPS to the computer are enclosed in a wiring harness loom, and it looks like it has not been taken apart or tinkered with.

When driving, it makes sense to me that the issue is TPS related. If the PCM shows little or no TPS voltage until the throttle is wide open, this kind of goes along with what it’s driving like because if you are driving it and it starts surging and bucking, if you press the throttle wide open, it clears up and runs great (until you let back off the pedal). I have had one mechanic tell me that he thought the PCM could be the problem, but I have a hard time believing that.

 

On the other hand, a used PCM and wire harness shouldn't be too expensive to give it a try.
Make sure you have no vacuum leaks anywhere.

 

I think I can get a new PCM from a local chain store for around $200...but I have been told I have to take it to a Ford stealership to have it programmed so the PATS ignition key will work. New $200 computer and it won't start, $75 tow bill to get it to the dealer, and who knows how much they'll hit me up for to re-program everything. I'm all ready in this thing too much as is. I have to ask myself the question "Should I keep on pouring money into it, or just cut my losses and push it off on somebody else".

But as I said, I have a hard time accepting the thought the PCM is bad. I have had dozens of vics and lincoln's with the 4.6 engine, and have had some really odd-ball problems....but never a PCM.

Thanks for the vacuum line idea. That was one of the first places I checked (and fixed a couple places). They all look good now.

 

I would like to offer a suggestion and that is, to get a hold of a wiring diagram for your particular vehicle. Once you have that you can trace the various systems and determine , for example, where the TPS gets its ground. I don't believe it is thru the PCM. The PCM gets it's signal from the TPS which I believe has it's own ground. I'm somewhat confused with your post where you stated the TPS response was smooth and yet a "mechanic" said it is not. It is entirely possible that both statements are true,however, that means the signal going to the PCM is interrupted somewhere and it could be in a harness somewhere. You know how to use a voltmeter and the diagram will make troubleshooting the problem much easier.

 

The mechanic said when (measured at the TPS) everything is smooth and good (and I have verified this myself with a multimeter).....but he said that when he looked at the PCM data with his diagnostic equipment, the TPS signal was not as smooth as when measured by back-probing the TPS unit itself.

For $hits and giggles, I tried driving it with the TPS disconnected and found it ran the same as it does with it plugged in, except it would not shift through the gears properly and would not downshift for a WOT....this tells me that the PCM is indeed seeing something from the TPS (but maybe not good enough to make the engine run properly).

I think I will try tracing down the wires from the TPS to the PCM and see what they look like. I just doubt I'll find anything. I just keep on thinking there is something everybody (including me) is overlooking on this car.

 

TPS ground circuit is provided by the PCM.

Re-test TPS signal with engine at operated temp. This time tap on the sensor repeatedly with the butt end of a screwdriver, and read the voltage as you open and close the throttle plate. This may reveal an intermittent glitch.

Intermittents in TPS may also be found by running a long wire from the signal wire to a multimeter, and road-test if your scan tool does not have this provision.

Make sure too the locking tabs on TPS connector are not broken. A connector worked loose can cause an intermittent false connection.

I'd be interested to know what are the 02 sensor cross-counts. Specifically Live Data PID's of 02S11 and 02S21. In closed-loop they should both ocillate between 0.100 and 0.900 volts.

The reason I ask is that at WOT the PCM returns to open-loop (always), and ignores 02 sensor input.

 

I have tried having someone tap and poke around the TPS while I had a voltmeter on it and it stayed smooth and varied from slightly less than 1 volt @ closed throttle and 4.5 volts @ WOT. I also tried moving the wiring harness and PCM plug while measuring the TPS voltage and found nothing. I know the engine was @ operating temp when I done these tests.

I would like to test drive the car for a few miles running in "open loop" (to rule out O2 related issues). Is there a way that I can force the car to stay in open loop mode?

 

From the readings you received it appears the TPS is functioning as required. The fact that the scan tool received a different reading at the PCM would lead me to believe the signal return wire(system grd) from the Pcm is either open or intermittent. Were all the ground wires for the PCM where they were supposed to be and making a good connection. Usually when we get a series of codes for all sorts of things , it involves the ground circuit. As mentioned Ford did have a problem with the printed circuit board in earlier PCM s with a cracked solder joint involving the signal return circuit , but I don't know if this year was affected by that problem. The most important thing you can do is to confirm that all the ground connections are where they are supposed to be and are good connections. Don't be afraid to condemn the PCM especially since everybody and his uncle have been in there.

Sometimes unplugging the coolant temp sensor will take the system out of closed loop.

 

+1 on the ECT sensor unplugged. That will probably keep it from going into closed-loop. It should also throw a code.

Also, as I'm sure the o.p. knows, there is a window of oppertunity to test drive the car, soaked to the bone cold, where it will stay in open-loop for a short period of time without unplugging the ECT. How well does it run cold?

Do keep us posted o.p.. This is an interesting thread.

 

Sometimes we force an issue to confirm function. Did you get any code when you disconnected the TPS ?