Engine problem: extra noise then drop in rpm

[hey-you]

Okay, I've not figured this out yet. Here is what happens:

After the engine warms up (2+ minutes) the engine will do the following:
1. make a additional roaring noise (like I've plugged a shop vac in) and I be imagining things but there is also a slight vibration.
2. if I'm driving, it all seems to go away, especially if I accelerate slightly, nothing really specific happens except the engine rpm doesn't seem to dropbelow 1200 if I'm coasting or moving slowly throught town.
3. if I'm stopped and the noise stops then it is immediately followed by a drop in engine rpm, last time it happened, it dropped to a stall.

Other info: about 10 months ago I had the transmission rebuilt, and I know that the vacuum line to the heater control valve has gone (since its winter and I have heat I was sitting on fixing it till spring). Finally, I know the oil sending unit is sticking on cold starts.

Any thoughts? I've not found much in my Haynes manual.

 

[hey-you]

okay, so the truck went to engine guys I trust in town... and I heard back:

Nothing showed up when they hooked it up to the computer, nothing wrong with the transmission either.

Best recommendation was to have a look at the fuel filter and plugs, sort of an indepth tune up.

Plugs were replaced summer of 05, so they should be okay, depending on what quality there were, but that was back before the truck was shipped to the UK and I don't remember.

 

[brendank]

Are you sure that the noise isn't just the fan engaging?

 

[30coupe]

Are you sure that the noise isn't just the fan engaging?

That was my thought too. That or the fan clutch is stuck so the fan is turning all the time at engine rpm. That wouldn't account for the stall though.

 

[hey-you]

I'd briefly thought about the fan but rejected it as I couldn't link it to the rpm drop. The truck is a 99, and 3 weeks ago is the first time this has cropped up.

At this point the truck now has a new fuel filter, new plugs and wires. I'll see how it drives as the week progresses. This morning I'd have never heard anything over the heater fan trying to keep the freezing rain off the wind shield. More to Follow.

Had a look at the spark plugs, once pulled, and they were done, even though they were changed about 3 years ago. At this point I'm entertaining the thought that driving the truck for 1.5 yrs on UK petrol (different octane and potentially different additives) may have played with the plugs and fuel filter, although I'd have thought that any remaining drops would have been well washed away by now.

 

[hey-you]

Okay. its been a couple of days now, and I haven't heard the vacuum noise again......

What it really was I have no idea, but it appears to have been stopped by changing the plugs. I'm glad its gone, but remain slightly baffled.

 

[hey-you]

Its been about a week since the plugs and wires were changed... and there it was this morning. Had been idling at a stop lighthaving driven 15k to work, and as I pulled away.... the extra roaring/vacuum noise came back. It stayed for 800m (1/2 mile) until I had a chance to accelerate and the truck shifted.

I know when I took it in, the comment was that there were no codes showing, hence the bafflement.

Anybody else have this problem?

 

[Earl43P]

"I know that the vacuum line to the heater control valve has gone"


That would MAKE a vacuum leak. Replace that bad vacuum line.

 

[shadetree]

"I know that the vacuum line to the heater control valve has gone"


That would MAKE a vacuum leak. Replace that bad vacuum line.

It could also be the source of the noise if the heater/ac is on. No blend door control. shady

 

[hey-you]

okay, sometimes it takes someone else to point out the obvious.... I'll have to see if I can get friday afternoon off. I'll let you know.....

 

[hey-you]

Update, okay so today I went and picked up a mechanic... I drove he sat there with the diagnostic computer hooked up looking for codes. After 40 min we got the roaring noise back, pulled over and noise quit before we got the hood open, managed to get it again and almost stalled the truck. No codes though.. nothing.

The vacuum line is working, seems that my disconnecting and reconnecting the line "fixed" it somehow (blockage or loose connection), enough vacuum to work the valve to the heater core.

So at this point, the mechanic has gone back to call his tech reps... but the best idea he had was that it might be with the fan, seems to be running at high speed continuously. (although I'm not sure what the difference actually looks like).

So... situation no change am still baffled.

 

[hey-you]

Engine is still making the vacuum cleaner noise.... only change so far is that I've found that if I accelerate once it starts after two shifts the noise is gone...... that and while driving today discovered that if there is no load on the engine when the noise stops engine rpm immediately drops like it does when sitting still (no surprise I guess, both being relatively no load).

Sadly, haven't managed to get out and get the hood open before the noise stops.... having read a few other posts I'm thinking of pulling and cleaning the MAF and IAC sensors.

 

[RedneKRangeR]

this sounds like the problem i have been having with my '88...
it was the fan clutch, the thing sounded like a screaming shop vac.
and it was killing my fuel milage.. when i would look under the hood it would be spinning full time even when it was cold, or running down the highway.
as it turned out the fan clutch was on its way out. i drove it for about 4 months with it sounding like this. then today acutally i noticed a vibration... so its finally gone tits up on me. the bearings that where in it where actually falling out and i could move the fan back and forth too..

i hope this helps
RedneK

 

[RedneKRangeR]

oh i almost for got, my truck was also doing the idle thing...
i would check out the iac, we had this problem with my moms 91 f350 it would randomly die and run erratically

RedneK