NEED DIAGNOSTIC HELP 91 F-150 5.0

So I have had this truck for a year or so now and it has always had this issue.

on a cold start up the engine will fire up immediately then stumble for a second, it will sound like the throttle is sucking in a large volume of air as the engine speed goes down into the <500 range. then it will promptly die. the only way to keep it alive is to manage the throttle myself and push it in when it starts to slow down, then with my foot held in place the engine will shoot up to around 1500-2000 depending on how far my foot goes in, it holds it place for a moment then starts to die back down. if I let my foot off too soon before the engine has time to switch over to using the O2 for fuel trim data it will die. when I do finally get it to run alright it will have what feels like a single-cylinder misfire that goes away above 1800 rpm.

the stalling issue will also come up if I am driving for a few miles and start to coast after letting off the gas. this happens regardless of engine temperature.



I have replaced the MAP, Fuel Pressure Regulator, all the plugs and wires, cleaned the EGR, checked voltage and resistance on IAC/EGR. so none of those would be issues I think. I will probably go recheck them again just to make sure though...

There is an Exhaust leak at the flange of the right header. I only noticed it when I was sea foaming the engine at high rpm.

Fuel pressure was normal on both pumps. the problem is also present when both pumps are used.

the timing looked okay last time I checked it, though I could be wrong as I did not remove the spout connector

there was a single code it was either 327 or 337 or a 328. I don't quite remember which because I lost the note that I wrote it down on and its no longer throwing that code. I did cut the muffler off a few months ago but it was throwing that long before then.

sounds like the EGR is open... unplug the vacuum to the EGR. the codes 327,337,328 are related to the EGR. Either the EGR is not closing tight or its getting a signal from the sensor to open incorrectly. If it idles ok with vacuum plugged off check EGR position sensor and regulator.

I have done this before when i was first trying to diagnose the problem.
hooked a vacuum gauge up to it and sucked it, the valve opened and there was no change at idle. you can feel vacuum pressure through the nipple when its removed from the sensor. I have also performed the voltage and resistance test on the sensor at the top of the EGR and all values were within specs. I even took the EGR out and cleaned it for good measure.
you can even observe the diaphragm moving when you rev the engine up while in park.

Sounds like a vacuum leak. Try spraying something around the vacuum lines and intake manifold. Or the tos is going crazy any live data?

Hi guys,

That seems to me like pcm is not going to the right place in fuel table when it has to go to open loop.

Are ECT, IAT and MAP signals getting pcm?

Stalling when coming to a stop and having to open the throttle to start the engine are symptoms of an idle air control type failure.

There are many videos in the chapter 20 playlist on YouTube, this is just one of them:


It may be beneficial to clean the throttle body and IAC passage, and check for improvement.

manually opening the EGR should have caused a change in idle, it should have killed the engine or at least made the idle worse. if the valve is not seating it will allow exhaust gas into the intake. remove the EGR and block off the port. this way you can rule out a seating problem.
cannot rule out a vacuum leak as the engine runs better at higher RPM. the problem is air to fuel ratio. when 02's are in open loop they go rich by default. this is not going to cause the stumbling or bad idle. if fuel pressure is good then its too much air entering the system. the open EGR or a massive vacuum leak is the direction I would work on.

Here is a video of the problem...kinda it wasn't stumbling as bad today for some reason and i was able to keep it alive


I plugged off the egr port and it seemed to go away, but when i unplugged all wires and vacuum lines from the solenoid it made no difference. That dosent make sense to me, perhaps the diaphram isnt closing all the way?

>clean the iac and throttle body.
i have done this before and it gave no change

the vacuum to open and close the EGR is controlled electronically, engine vacuum controls how far open the EGR goes. If things got better with the port blocked off, the EGR is not closing all the way. Idle is still rough because you may have a vacuum leak (did you plug the EGR vacuum line with the port blocked off?). now you can work on a possible vacuum leak. with engine running you can use water about the engine and note any idle change. take a plastic water bottle and poke a hole in the cap then squirt water on the area between the heads and intake. smoke test is best if you have it otherwise pain staking check of all the lines.

Will do, ill tell you if i find anything.
Thanks

Okay, i couldnt find any large noticable vacuum leaks using a squirt bottle or by blowing into a hose with a vape.

Where would you turn next?

And in case you were thinking that the video was with the egr port plugged off, its not, that was with the whole thing assembled.

The engine ran pretty good with it the hole plugged up, but i couldn't imagine that the egr valve itself would be stuck up because i was able to open it myself by using a hand pump....

guafa wrote: Hi guys,

That seems to me like pcm is not going to the right place in fuel table when it has to go to open loop.

Are ECT, IAT and MAP signals getting pcm?

i dont know how should i check that?

you can move it up, but it appears that its not going down all the way and sealing off. the pentel could be down but not sealing. the pentel itself my be damaged, warped, bent, cracked from heat and age. don't go buy a new one. fully inspect and test it first. slight damage or mis shape will cause a leak and allow exhaust gas to enter the intake at the wrong time.

if the internal spring for the pentel has weakened over time, this will allow the exhaust pressure to push open the pentel and allow exhaust gas to enter through the EGR and into the intake.