Need some Ford help. 5.4L cold starting trouble

[Russell]

Dad has a 98 Ford F250 with a 5.4L Trition engine in it. MPSFI with coil on plug ignition.

The engine won't start from cold, and has to be cranked for 20 - 30 seconds before it will finally fire and start. When it does, it usually revs up to around 1200 rpm, then stalls out. It does that 2 or 3 times before it will finally run. Any attempt to touch the throttle will kill it immediately. Any attempt to put it in gear results in the same. When it does run cold, it has a noticeble lean miss to it, and the exhaust smells strongly, like a lean carbed engine would.

I've got an OBDII scanner that I've hooked up to it, and all the sensors seem to be reading normal. IAT and ECT both read within a few degrees of eachother before you start the truck, and are close to ambient temperatures. The ECT reaches 195F as expected as the truck reaches operating temperature. The mass air flow meter ranges from 1.7 lb/min at 1000 rpm to 7.5 lb/min at 4000 rpm. The ambient air temperature is approx 5°F.

I plotted the RPM vs mass air flow rate, and it does make a fairly linear line, so I think that the mass air meter is alright. The MAP sensor reads normal ambient as well.

Truck has brand new plugs / plug boots (had the issue before) and it runs & starts perfectly normal when it is at operating temperature. I just also just replaced the fuel filter & the injector filter baskets, and am fairly convinced that it is not a fuel supply issue.

Anyone have any ideas of what might be ailing this truck? I can't figure it out, and usually don't have so much trouble with EFI issues

 

[Fordum]

Well, as you know, these trucks are running blind until they get up to op temp. They have no way to tell how much gas is getting into the cylinders until the O2 sensor lights off.
So, they look at the sensors that do work, make some assumptions, and open the injectors the number of milliseconds the burned in lookup table tells them to.

So, either the computer is glitched and not looking at the right part of the table, which usually corrects its self the next time you crank it, a sensor is lying to the computer, or one of the assumptions is wrong.

I do not have any of my books handy that would tell me what the sensor readings should be right now, so I can't comment on your readings. But, you might consider that just because the mass air flow rate tracks with RPM, that does not mean it tracks with the right value. It could be a few hundred ohms off all the way across the board.

However in this case I think you are going to find its a fuel issue. As long as the engine is in open loop, it is opening the injectors the amount it thinks it needs for a certain fuel pressure.
As soon as the O2 sensor comes online, it sees the problem and holds them open longer.
As long as the time is within its permissible range, it will not throw a code.

I would put a fuel pressure gauge on when its cold and check it when it is running.
I suspect that the fuel pressure regulator is not holding the amount it should, or the pump is sluggish when cold.
If you don't have a gauge handy, you can check to see if the regulator is having problems with the cold by heating it with a hairdryer before you try to crank it.
A gauge is better, because it might not be a cold issue, just low pressure all the time but the computer can compensate.

Also, if the pressure is ok, try the hairdryer trick on the other sensors, map, etc. Not the temp sensor though, for obvious reasons.

J.

 

[Russell]

Well, since the fuel pressure is the only thing I don't have the tool to check on I'm guessing that is going to wind up being the issue.

Just seems like it would take a fair bit of compensation on the PCM's part to make up for that weak of a fuel pump without any running issues at all when hot. I can take that truck to wide open throttle and she doesn't miss a beat.

 

[Fordum]

You would be surprised how long it can hold those injectors open. But its running so good when warm, is what makes me suspect a low fuel pressure problem that goes away when the truck warms up. I don't remember where the fuel pressure regulator is on that engine, but it is on the back side on most of them. It should be at the end of the fuel rail regardless, since it does not restrict the fuel pressure or flow coming in, it just controls the flow back to the tank.
If it is sticking when cold, try the hair dryer trick and warm it up. Pressure gauge is better, but that is a quick and dirty way to check until you borrow a gauge.
I doubt that it is holding low pressure when warm, or you should see something when you rev it up.
Of course, if its a sluggish fuel pump or booster pump, heating the regulator won't help regardless.
Check with some of your national part store chains in your area. A lot of them will loan some testers.
J.