Hello to all! I have an old CL&P bucket truck that we use at our fairgrounds for elevated repairs. Late summer I started having throttle problems with it. Now, if I step on the gas, it dies, unless I let off the throttle. Then it recovers. If I feather the throttle, I can get full throttle. I have replaced the fuel filter, checked fuel pressure, it is within specs. I checked out the chk eng light. Running, I get 10,12C, 52c, 53C. I checked the ecu wiring to the TPS sensor, all within specs, and load tested ok. I replaced the TPS sensor. Still the same. I pulled the ECU and sent it out. They found a badly corroded circuit board, like the unit had been underwater. I said, OK but not on this unit. The ECU was rebuilt. Same problem on re installation. Now I have codes 10, 31,54. I have checked the unit, don't see any vacuum leaks.Intake is clear. EGR valve is closed. Any suggestions?

 

What engine is it?

The computer needs to added extra fuel when you press down on gas pedal quickly, so TPS was a good guess, as thats why it is there, to give computer the "heads up" to add more fuel quickly, a MAF or MAP sensor reacts to slowly to changes

Computer would also change spark timing instantly for the richer fuel mix, if you have a TFI distributor spark system that would be done via the SPOUT wire
If distributorless then the ICU(ignition control module) or EDIS module should get the "change park timing" signal from computer

Vacuum leaks
After engine is warmed up
let it idle and unplug the IAC Valve
RPMs should drop to 500 or so, barely running or engine may even stall, either is good it means no vacuum leaks
If RPMs do not drop then there is a leak

 

Sorry, I thought I had the engine listed. It is the 7.0l engine. I'll have to find the IAC valve. I don't see a MAP sensor on the intake. There was a code for TPS voltage high. 52 or 53. That was why I replaced the sensor. The wires checked ok. I will take the gauge and check the vac. drop tomorrow.

 

429 nice, but a heavy truck, lol

In 1991 it probably uses speed density so a MAP sensor, usually not on the intake maybe on firewall, but will have a vacuum line to the intake, make sure that hose is not leaking

 

I remember a line off the intake to a sensor on the firewall. I'll check that today. Thanks

 

I checked the line to the map sensor, no leaks. I checked the intake system, no leaks. The PVC has strong vac. When I remove the line the rpm drops about 150 rpm doesn't stall. This has an intake power valve below the air intake horn. When it is unplugged, idle drops, unit barely idles. When throttle is applied, same problem.
I talked to the parts man handling the ecu repair. He said the circuit board for the TPS sensor was what was bad and replaced. A couple of additional diodes were replaced and all solder joints checked. The repair service said they ran full function tests, so it should be ok.
The MAP sensor has to be ordered. Unit is too old, nobody stocks it

 

I replaced the MAP sensor, rechecked the vac line. When I ran the engine, vac is about 12.5" steady. Push on the throttle, vac drops to 0 or below. Returns to 12 when I let up. I had to remove the intake tubes from the air filter. A fitting broke loose on the junct. for the intake to the air compressor, while I was checking the air valve on the intake. I ran it with the tubes off, and when I move the throttle, intake sucks a good amount while the engine is faltering. When I let up, it returns to idle.

 

Thats pretty low vacuum for a gasoline engine, expected is 18"-21"

Even in Denver, 5,000ft elevation, 16" would be lowest expected

Drop to 0 is not good, below is BAD, need at least +1 or +2 to keep air flowing into cylinders
Quick drop to 0 is OK but key word is quick, vacuum should return quickly as well

Could be blocked exhaust, but engine does REV slowly correct?

Good place to read up on Vacuum testing: https://www.gregsengine.com/using-a-vacuum-gauge.html

No matter what "they" add to the outside of a gasoline engine its still just a self powered air pump, so testing it's pressure(vacuum) can tell you alot

 

OK, thanks. I'll check it out.

 

For vacuum leaks I use a spray bottle with soapy water in it, cleans the engine and if its sucked in RPMs will change, lol
And its NOT flammable, so no OOPS on a hot exhaust

And its pretty much FREE, I am cheap

 

Next good day(no rain) I'm going to check the EGR valve. I've resealed just about all the vac. connections I can find other than that. I think that needs a closer look. If the valve is starting to fail internal, that would explain the large vac. leak. I am cheap too. A soap bottle is a good trick. I just happen to have one there.
When I crack the throttle, the engine picks up quick like it's supposed to. I have to let up the throttle so it won't die. If I hold the rpm hi, the engine starts to break up and it's backfired a couple of times. Not through the intake, out the exhaust. I don't think the exhaust is blocked, but it could be possible.

 

I blocked the EGR valve, no change. Still 12" vac. I have gone over the intake with soap, unable to find any leaks. I don't know where to go from here.

 

Hello again! I am still working on this 429 engine. I have not been able to find the fuel problem. I find a constant code 54 while KOEO. and running also. I get a code 31 and a code 10, but those aren't listed in the code book with the reader.
The ECU was rebuilt 11-2019. I checked the wiring for the intake manifold temp switch, and it is good. I had replaced the sensor when it showed open. I rechecked the sensor when the code came back. The sensor shows 29.9 on the 200k scale. The resistance drops when I heat the sensor with a hair dryer. I think the resistance is high. Am I correct?
I checked the voltage on the input wire(rd-blk) and it shows .16v from the ECU. Isn't this too low?

 

I double checked the vacuum, found I was using the wrong port. If I use the rear port, I get 18" vac and is is steady

 

There are/were no codes with "0" on the older ECUs(PCMs), so disregard 10, the 3.8l computer did have some "0" codes, but not what you have

Ford 2 digit codes here: https://www.therangerstation.com/tec...gitcodes.shtml

31 would be expected, all the time, if you deleted EGR valve, it means there is no signal coming thru valves position sensor

54 is for the intake air temp sensor, its a 2 wire sensor, it can be on the upper intake manifold, or the air filter box or air plenum hose(between air filter and intake)
Has 2 names
IAT - intake air temp
ACT- air charge temp

Often looks like this: https://i.ebayimg.com/images/g/5P8AA...bIU/s-l300.jpg

Can be screwed in or a friction fit(pushes in), but it will be in the air stream

 

Hi Ron,
Thanks for your reply.
The EGR valve was blocked off, but not removed. As far as I can tell, it was moving properly. I found the IAT, and checked it. It was showing an open circuit, and I replaced it. After running again, I still showed an active code for the sensor open circuit. I rechecked the new sensor and had a 29.9 reading on the 200k ohm scale. I heated it with a hair dryer, and the resistance dropped. The reading did show a change. I am wondering if that resistance is correct? I also checked the rd/bk wire at the sensor, and showed 1.6v from the ECU. Is that voltage low?

 

I don't have a wiring diagram for your vehicle and year

But Ford used a 5volt power supply inside the computer, shared by most sensors, on a grey/red wire in Rangers
So check "red/black" wire at ECT, EGR, and IAT, key off, see if they all are connected, 0 ohms, should be
Then check voltage at all 3, should be 5volts, BUT..............its a pull up circuit so some volt meters need to have polarity reversed

So low voltage could explain the 2 codes, but should have ECT code as well unless it a splice issue and ECT voltage is OK

ECT and IAT are basically the same OHM reading by temp, ECT of course gets hotter

IAT here: https://www.aa1car.com/library/air_temp_sensor.gif

ECT here: https://www.explorerforum.com/forums...art.24365/full

 

Thanks Ron,
I got a pin-out diagram from local Ford parts to check voltages when I had the TPS problem. I will go up in the AM to check sensor voltages. Each sensor has a separate feed. There are no shared feed wires. I figured there should be a voltage present on the sensor input wire. There is none on the IAT sensor input. I will double check the others, and at the ECU. Update tomorrow

 

TPS should also have the red/black wire

Each sensor has its own "signal wire" back to computer, but they should share the one 5v power wire, like all the other systems in the vehicle "share" the 12volt power

 

Thanks Ron. I think that would be the red wire, reference voltage output.Correct? I will go back up and look again. Is there anyplace that stocks ECUs from these old CL+P trucks? I checked with the parts house that sent out the ECU for repair. It is 30 days past warranty, and they won't budge on covering the repair.

 

It would be an unusually failure for the 5v power supply in the ECU to drop to a lower voltage, 0v would be more likely failure
So I doubt its an ECU issue, corroded connection or splice would be most likely
Also the codes would be different, the 5v power is used to compare the return voltage, the difference is that sensors "signal", so if the 5v power supply was 1.6v then signal voltage would be compared to that, not 5v

 

These are the pin-out diagram for the ecu plug sent from Ford. I checked #26 and found 2.4 volts. I checked the temp sender, and the coolant temp sender. Neither shows 5v. Can you see any other than # 26 that would be supply voltage?



These are the pin-out diagrams I was sent from Ford

 

Pin 26 is the 5v reference on all EEC-IVs, so I guess thats the problem, but.................unplug all the sensors using the 5v, one sensor could be pulling it down, but long shot

Does read like ECU is the issue

Pull out the ECM and open it, there are 3 capacitors that can leak, blue ones, easy to spot, I just had to replace these 3 in my 1994 ECM, 2 were bad
Cost about $5 to fix it
My issues were poor running, intermittently, and odd codes sometimes, also fuel pump would run full time, key on, now and then

 

I don't have a wiring diagram either. I will try my Ford dealer next week. I checked the sensors that I can see, and none of them have the red #26 wire. I unplugged the temp sensor, and the IAT sensor, no continuity shows between the two. I will check other sensors I can find. I can try popping the cover, but how do I tell if the capacitor is bad? Where can I find new ones? Radio shack is gone...
Thanks for your input.

 

Might want to check these out

OEM 1991 Ford FB600 Fb700 F800 FT900 Electrical & Vacuum Troubleshooting Manual
https://www.ebay.com/itm/OEM-1991-Fo...8AAOSwH-Ve9Eb6

or maybe this

1991 Ford F- FT- B-600 700 800 Truck Service Shop Repair Manual
https://www.ebay.com/itm/1991-Ford-F...8AAOSwh5Re8r5i