Vehicle: 1993 F350 crewcab, 5.8L, ZF 5sp, SRW, minimal options, way too many miles.

The truck's been fine for years, then all of a sudden yesterday it stalled while taking an exit ramp, and just as I started to decelerate on the back of the gears (i.e. downshifting), I could feel in the shifter lever an interesting bucking as it jerked side to side.

Since I normally upshift early and downshift later than most, I often get pretty close to idle and this time it stalled on this exit ramp. I thought nothing of it, thinking maybe I "overdid it", and restarted it and continued along like nothing happened.

However, anytime I push the clutch in and the engine returns to idle, the engine "dives" far below idle, then surges far above idle to recover, then dives back below idle and stalls. Restarting it is fine, except idle is a little lower than it's usual 900 RPM, though upon restarting it the idle is stable - no hunting, diving, etc. It's just lower - about 750-800 RPM.

While playing with this on the way home last night I noticed that it feels to have an overall lack of power, but that's a judgement call, and might of been because it was two in the morning and I this anomaly occured for the first time on the way home when I was tired and not caffienated.

I also noticed that while decelerating in gear, the "rumble" I've always had is minimal to sometimes not even there.

The engine light is not on, and there were no codes to "blink out". I did that last night when I got home.

Because the behavior of the vehicle changed suddenly, I'm assuming something "broke" rather than slowly faded away.

I tested it this morning and it started up fine, but the cold idle was noticably higher than it normally is for the first start and while it lowered as quickly as it normally does (30 seconds or so), it landed and stabilized quickly below 900 RPM.

Other than replacing the obvious - cap, rotor, fuel/air filters, where would you start? The cap, rotor and fuel filter are basically new - less than 1500 miles and the air filter isn't new but is clean. I just checked it.

The sudden lack of "rumble" while decelerating leads me think maybe it's the TFI module, and the idle diving up and down after deceleration, then stalling, leads me towards thinking maybe it's the IAC valve is sticking.

Though, this sudden change in behavior is "dead nuts" consistent, and I've never had an IAC valve go bad where it's this consistent.

The distributor housing is tight on the outside, and the rotor has no play in it. Maybe it "jumped" ?

Not that it should matter, but my F350 crewcab thinks it's an E350 dually ambulance, EEC wise.

It is 350 miles over it's due time for an oil change, but I don't see how that would cause this specific behavior as precisely and consistently as it's occuring.

Any ideas before I go outside and start randomly wiggling things? I *know* this is going to get worse, and I won't be able to compensate easily enough.

Thanks in advance!

 

Three things come to mind IAC as you mentioned, TPS could be going bad, a small vacuum leak may be rearing it's ugly head. A sluggish IAC and small vacuum leak are big contributors to a hunting idle speed issue like this.

 

https://www.ford-trucks.com/forums/1...-for-idle.html

Another possibility? Although I think an EVR related issue would cause some sort of EGR code...

 

I did some diagnosis this morning, and here are the results/observations. I'm going to share all the detail in case it's interesting or useful to someone.

The engine has far less vaccum than would be typical, and while I assume that's due to the extreme mileage I have on the engine obviously I have to rule out a vaccum leak first. I attached a vaccum gauge, took a reading, then yanked off the vaccum hoses and the brake booster hose, plugged them with my fingers, and as the engine settled down, stumbled around idle, I had the same gauge reading as before. So, I do have a low-vaccum problem but seemingly that's not from anything leaking.

I then took off the TPS and blipped the throttle plates and the engine sounded very different, stumbling on it's way back to idle. Adjusting the TPS position without moving the throttle plates choked the engine on fuel. So it seems the TPS sensor is working because by adjusting throttle plate position and the TPS position individually, I can easily make the engine very rich and choke, or very lean and unhappy. It might not be in perfect condition but it's at least working.

Testing the IAC is easy - with the engine idling the IAC should be completely open so removing the connector will allow the spring inside to push it closed, thus stalling the engine. I yanked the connector, and viola, the engine stalled.

Then, I took a TFI module I had in my junk drawer and swapped it with the one in my truck, leaving it to dangle since I was only going to do this for a minute, blipped the throttle, and the deceleration of the engine didn't rumble in the exhaust and of course, the idle once again dipped below 900, jumped above, then dropped below and stalled. I plugged the harness connector back to the original TFI module.

I started the truck and listened to the lower than normal idle, as it waved between 700-800 rpm. I sipped my coffee, staring at all the brittle vaccum hoses, the mostly original things those hoses attach to, I'm listening to pulley whine in several places, the annoying valve tap I've had for at least a decade, and for a moment, was impressed this thing still ran.

Then it happened - I heard a click followed by a hiss, and the idle dropped and the engine stalled, then another click - passenger side. I started the truck again, ran around to the passenger side, and listened carefully. It took about 10 minutes but I heard the click again, the hiss, and the engine stalled.

The EGR valve. I unplugged the connector and started it back up - while the check engine light came on immediately, blipping the throttle sounded healthy once again, I had a very loud rumble during engine deceleration in the exhaust, and when the engine hit 900 rpm on the dot - it maintained it perfectly - no hunting, no stumbling, nada.

I'm assuming the positive result is because something is borked with the EGR valve, and the sudden "renew" of how the idle should be is not because the check engine light was on and the EEC switched into "limp mode". I'm making that assumption because on one occasion I had to drive home in "limp mode" and honestly, I wouldn't even call it "limping", it was more like miserable dragging.

So, I think I'm going to make an EGR block-off plate since none of the local auto parts stores has an EGR valve in stock, and that will allow me to drive to a customer today.

Just wanted to share what I've done thus far.

 

Nice work, thanks for the details.

 

Sure thing. I figure the results are probably as important as the question. After all, I'm not the only guy with an old truck

I do have to replace the EGR because I have to pass NJ emissions but in the meantime, a block-off plate will allow me to drive around "now" while I wait for the thing to arrive.

I decided to buy it mail order simply because it's half the price, even with shipping.

 

^^^

What he said! Always good to know results. There's a lot of threads that come up in searches on here that end up being incomplete. Hopefully this helps someone!