1988 F250 5.8 351

Need some troubleshooting logic help. My truck started having issues starting in the morning when ambient temp was above 70. Had to crank on and off at least 3 times to start. Searched other past treads and then replaced ECT and IAT sensors. Still no change. Here is what I've done/checked.

Fuel pressure at rail = 38 psi after one key on pulse in the morning after sitting all night. Checked reference voltage KOEO with ETC harness removed = 4.7v. Checked resistance across ETC sensor with harness removed = 22K ohms. Checked continuity out of pin 7 from computer through ETC sensor and back to computer at pin 46 = all good. With KOEO and harness removed I jumpered from the incomming 4.7v reference wire to the sensor and read on the outgoing sensor pin = 3.0v. Sensor reading and voltage/resistance through sensor look good. Here's the strange thing, when I replace the sensor harness and back probe the signal return wire all I see is .07v. I removed and disassembled the computer looking for any obvious burned component or connection on the circuit board but everything looked ok. WTF! I don't get it.

I'm thinking the computer is not functioning correctly at least in regard to this sensor and is flooding the engine when starting thinking its cold outside.

Before I replace the computer I wanted to see if anyone on this forum had any thoughts.

Thanks in advance.

 

The signal return wire should always be low (around 0 volts). It's just a dedicated sensor ground.

There are other things that could cause hard cranking, such as a bad MAP sensor, bad TPS sensor, or even a failing TFI/PIP module.

How does it behave, does it just not try to start at all, or does it run real rough and won't stay running?

 

Well then it's (the signal return wire) doing what its suppose to do. Where is the resistance or voltage variation read by the computer? Is it on the signal wire?

The issue is not hard cranking like a starter issue. It cranks great. It's rather like a flooding situation at initial startup. I cranked it 20 turnovers no problem but no starting kick either. I could smell gas after that crank. Truck always has had a sick smell to the exhaust like to rich much of the time. Truck starts eventually, its just a real pain especially on warm mornings.

The issue that started me trying to fix the hard start when warm was when I was pulling a trailer up hills yesterday with throttle 3/4 to full I was getting mini stuttering or pulsing like lack of fuel or incorrect ratio. Some other past threads seem to indicate that a faulty ECT or IAT sensors would show both symptoms. The ECT sensor was original to my truck so I thought I'd replace it and the IAT and get a easy win. It's never easy on these trucks! I just replaced the fuel filter a month ago so I discounted lack of flow as a possibility. I had a thought about other sensors as being possible culprits.

I just cleared the codes so I'll use the truck today and check to see if anything shows up.

Thanks

 

To your questions: Once the truck starts it runs seemingly ok and will start anytime during the day with almost no problem. It always has a slight shutter at idle initially but not so much that it ever stalls. It's the overnight sit and initial startup as well as the hill issue I mentioned before that's the problem. I'll check the signal return wire off the TPS tomorrow morning KOEO and see what it's putting out.

 

The actual value read by the computer is on the "Signal" wire. Signal return is just Ford's weird way of saying sensor ground.

During the hard/no start condition, you should check for spark, or try seeing if a bit of starting fluid down the throttle body gets it going.

 

Is the assumption that there may be no spark in the initial hard start condition?

 

Yes, that is correct.

 

Thanks. I'll test tomorrow morning as the "no start" condition only happens first thing in the morning.

 

Checked for spark with a timing light on #1 spark plug wire. Appears to blink normally during cranking with no start. Tried starting fluid and it fired instantly.

Computer code shows #32 - EVP under lower limit. Fooled around with the EGR and the EVP for a while but non conclusive. 5v coming in, .28v going out. Plunger of EVP reacts smoothly when vacuum is applied. Resistance goes down as more vacuum is applied.

Checked the fuel pump relay and the PCM relay while idling. Both are warm. If I lightly tap on the FPR I get a RPM surge. Don't know if this is relevant or not.

 

If this is easily repeatable, I would start by changing the fuel pump relay and verifying the wiring and connections on the FPR relay socket.

 

I was experiencing stuttering acceleration pulling a trailer from dead stop in 1st. Got home and checked the TPS output. Looks good with a smooth voltage increase from .87v to 4.5v at full throttle Changed out the fuel pump relay. I could not get the new relay to surge the engine with similar tapping. That's promising. I'll see what happens tomorrow AM.

 

Same issue this morning. No start until starter fluid is sprayed. So the injectors are not opening until the truck reaches idle RPM, then the computer is running the whole show just fine. It seems to me that some sensor in the distributor is sending a signal at cranking RPM to tell the injectors to open and let fuel in. Why this only happens after an overnight sit is strange but it's consistant. Seems like a new distributor/ignition module is next. Thoughts?

 

The same sensor controls the injectors and spark. If it starts with starting fluid, then the PIP sensor is likely fine.

 

i read on a different forum that a weak PIP signal could cause spark but no injector pulse. I have a remanufactured distributor coming. Was under a warranty so I'm going to try it without the expense. Worst will be I'll have a new part with a PIP sensor and a ignition module that are fresh. The only other idea I have is to check for injector pulse while cranking and see what's what.

 

Let us know how it goes.