Cold high idol and tach not functioning properly

[Nick_Rego]

Hey guys, so it's been awhile.. The 91 f350 has been running great not really any drivability issues since my last batch if issues. I've been daily driving it too work and my parents I've only put about 400 miles or so on it and I noticed eveymoring now when you first start up and its cold it idols right around 650 rpm but as soon as you give it any fuel it bumps it up too 850 -900 ish rpms. And it keeps this as the idol when stopped until about half way to work which isnt far and I'll here a click like the glow plugs make when they turn off and my headlights will flash and the volt guage shows a small change happen. It hasent really affected how it drives much that I've noticed but I have never witnessed this truck doing this before my last batch of issues. Is this a high idol for warming the engine up or are my glow plugs staying on?
I will say lately I've had about every other time I'll turn the key and I wont hear anything the wait to start light comes on but none of the usual sounds the glow plugs make. But the truck starts right up , and the volt guage shows a decent drop still even with no sounds and it does click off so I assume they are still working.
The second issue I just noticed this morning on the way to work which is the tach now when at idol sits just above 0 until you accelerate or rev the motor and if your in park you can rev it and the tach will go back to the proper idol but then after a few seconds start to bounce and then drop back to 0. That is with engine running by the way. After a quick google search alot of people recommended just replacing the tach sensor before I do however I was wondering if you guys have any other suggestions too look into?

 

[MtnHaul]

Personally I wouldn't worry about the tach unless you have an automatic trans. My tach hasn't worked right for about 2 years, although just yesterday is seemed to be reading accurately again. The RPM difference you mention is the high idle solenoid for warming up the engine. The click you mention and headlight flash I don't know what would cause that. You could try disabling the high idle solenoid. Mine is broken so it's naturally disabled but there should be a wire from the solenoid that connects to a sensor in the block--disconnect that sensor and see what changes.

Also, you mention "last batch of issues"--were they electrical issues?

 

[MtnHaul]

What part of CA you from? I feel like I've seen your truck around town. I live near Santa Cruz.

 

[DaveBen]

The other thing about diesels is they need to be driven and driven hard. You need to take it out and get it hot. They are not grocery getters. They are made to haul loads. You will begin to have other problems if you do not get it hot.

 

[MtnHaul]

The other thing about diesels is they need to be driven and driven hard. You need to take it out and get it hot. They are not grocery getters. They are made to haul loads. You will begin to have other problems if you do not get it hot.

On a Mercedes diesel site I came across the concept of "The Italian Tune-Up". Now this is a highly technical procedure that involves finding an area of minimal traffic and driving your vehicle as hard as possible to uhh, clean the pipes out so to speak. Hills are your friend in this goal!

 

[Cubey]

Personally I wouldn't worry about the tach unless you have an automatic trans

If it's a C6, it doesn't need a tach sensor.

 

[DaveBen]

Drive it like you stole it!

 

[gandalf]

On a Mercedes diesel site I came across the concept of "The Italian Tune-Up". Now this is a highly technical procedure that involves finding an area of minimal traffic and driving your vehicle as hard as possible to uhh, clean the pipes out so to speak. Hills are your friend in this goal!

Funny you should mention "The Italian Tune-up". I did an instant flashback to a time when I was driving the Autostrada, going from Venice to Rome. I was doing a sedate 65 (my inlaws were with me), when I heard what sounded like a little chainsaw, and a small Isetta 300 passed me. That man was just going balls to the wall. Fifteen minutes later I passed him, pulled off to the side of the road with his hood open. Half an hour later I again heard the small chainsaw sound, and there he was again, going flat out. He had apparently stopped to let his engine cool.

It must be an Italian thing.

 

[chillman88]

when you first start up and its cold it idols right around 650 rpm but as soon as you give it any fuel it bumps it up too 850 -900 ish rpms. And it keeps this as the idol when stopped until about half way to work which isnt far and I'll here a click like the glow plugs make when they turn off and my headlights will flash and the volt guage shows a small change happen

That's completely normal. While you're waiting for the glow plugs, push the throttle all the way down and then let off to set the high idle, that's what's happening when you "give it some fuel" but you're supposed to do that before you actually start it.

That click and voltmeter issue is the high idle and cold timing advance shutting off because the truck has reached around 130 degrees I think is where it shuts them off.

 

[quickster]

Like Chillman said, that's normal. Don't worry about the tach when cold either. My tach won't register till the heat comes through the vents and warms the dash up. Been doing it for years. These trucks are getting old- some things just don't work well until warmed up, like my back.

 

[Nick_Rego]

Also, you mention "last batch of issues"--were they electrical issues?

It is an automatic and I had some glow plug main wire issues and battery issues.

 

[Nick_Rego]

What part of CA you from? I feel like I've seen your truck around town. I live near Santa Cruz.

I'm from Nipomo before me The truck was up north a bit in Atascadero and then before that I belive it was even further north somewhere since new.

 

[Nick_Rego]

If it's a C6, it doesn't need a tach sensor.

It is an E4OD

 

[Nick_Rego]

On a Mercedes diesel site I came across the concept of "The Italian Tune-Up". Now this is a highly technical procedure that involves finding an area of minimal traffic and driving your vehicle as hard as possible to uhh, clean the pipes out so to speak. Hills are your friend in this goal!

I've gotten on it a few times to clear the pipes out. I should mention it is pre smog in CA which means it has a 4" down pipe off the turbo and then comes down about just past passanger door and then angles tip out so shes loud lol. Also the truck has 410s out back so when I do step on it feels like it moves anyway. But I really baby this truck I honestly dont need it for what I do Which is literally 10 mins to work and 10 mins back. But occasionally I'll go run around town and mabye take it up the atascadero grade here on the central coast of CA but diesel is also expensive so I've been really trying not to drive too far and I'm really not wanting to over boost this thing since it's a non wastegated turbo. I only drive it becasue I've always wanted one and it's a fun truck to drive and it gets much better mpg than anything else I have minus the wifes car. My other options are a 68 f250 which gets like 7 and a 69 falcon which gets about 13 mabye on a good day? Anyway this truck so far has consistantly gotten 14 mpg which for my standards is nice lol. Even though the wifes focus gets like 30... Anyway I'll have to go take her out and open it up a bit see how she does.

 

[MtnHaul]

and a small Isetta 300 passed me.

I can't stop laughing just picturing an Isetta going flat out and passing people.