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RPMs drops while driving cruising speeds


wicsmith

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No codes, yet. Doesn't store between starts.
While driving in 3rd gear or 4th for a period of time the Cluster will show RPM's drop by 1k to 1.5k ability to maintain speed isn't altered. No other notice of issue until come to complete stop. At complete stop the RPMs settle at just below 500RPMs engine is shaky and the Check Engine light comes on. Accelerating from stop is now labored and struggles to get to speed.

Holding the RPMs high through a few gears and the RPMs return to normal and engine runs right for a few miles or less then process repeats itself.

If I notice the RPM's drop before coming to complete stop Holding high RPMs again corrects the issue for a short time.

Normally this only happens while driving. However more than once cranking cold and sitting while running it has occurred.

This occurs every drive longer than 10 minutes.

What are your thoughts? What can cause the RPM's to temporarily tank?
 


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RonD

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Couple of things come to mind.
Exhaust restriction causes pressure to build which limits new fuel from coming into engine so power/RPMs start dropping.

Fuel pump is going out, or fuel pressure regulator, as pressure in the fuel rail drops you lose power/RPMs.
Fuel pressure regulator is on the fuel rail, it will have the fuel return line and vacuum line.
Remove vacuum line and smell it for fuel, should be no fuel there.
Plug the vacuum line, start and drive the truck, that should keep pressure up, see if you have the same problems.

Get a fuel pressure gauge, '94 should have 35-40psi
 

wicsmith

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Thanks for the quick reply. I have noticed the trucks power surging a bit while cruseing also. So fuel regulator or pump might be a good start. I was thinking perhaps IAC valve. Since it had the low idle at times.
 

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Torque converter clutch sticking could do that.
 

wicsmith

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Remove vacuum line and smell it for fuel, should be no fuel there.
Plug the vacuum line, start and drive the truck, that should keep pressure up, see if you have the same problems.

Get a fuel pressure gauge, '94 should have 35-40psi
Smelled Vacuum line and did smell fuel. Plugged vacuum line but same problem occurred.
 

RonD

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Replace FPR(fuel pressure regulator), if there is fuel in the vacuum line it is bad now, and will get worse.
 

wicsmith

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Replace FPR(fuel pressure regulator), if there is fuel in the vacuum line it is bad now, and will get worse.
Replaced FPR today and took it for a drive. Response from the gas petal when coming off of first was better but the rpm's still dropped after a period of driving.

Next step I guess is to test the fuel pump.
 

RonD

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Replaced FPR today and took it for a drive. Response from the gas petal when coming off of first was better but the rpm's still dropped after a period of driving.

Next step I guess is to test the fuel pump.
I would get/rent a fuel pressure gauge.

You should see 35-40psi with running engine
Raise RPMs to approx 2,000 and hold it there, see if there is a slow drop in pressure.
Without a load on the engine it is a tough test to read.
 

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Got some Fuel pressure readings finally.
Key On Engine Off: 18
Key On Engine Running: 28
Reved Engine: 36
2000 rpm: 30
Ending de-reving: 25
Sounds low from what RonD said I should see 35-40
All of these numbers where pretty consistent during the test. No fluctuations.
 

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Yes, the pressure is too low and that will cause it to loose power...surprised you didn't get some bucking in there as well because that's what my Tempo did every time the fuel pump started to go south.

Now, the matter is simply do you want to drop the tank or remove the bed...I suggest removing the bed because that allows you to get really deep into your truck with things like replacing rusted bed bolts, finding and repairing rusty box areas, incorporating several friends or relatives to assist on removing the bed, and imbibing several cold beverages in the process...
 

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Now, the matter is simply do you want to drop the tank or remove the bed...
Watched a Youtube Video and that individual removed the bed. I plan to do it that way. I am curious on how to best clean out the tank good once bed is removed. Siphon? Cant really swish it around and dump it if tank isn't removed. Thoughts?
 

Mark_88

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The top of the tank is a fairly big opening and you could easily see inside if there is debris in the bottom with a flashlight...do not use matches or candles...they don't shine enough light...

I would say siphoning is probably the safest way to do it but only if you have a hand siphon pump...you can pick them up at hardware stores for $5 and good to have for other things as a just in case...I'd post a picture of mine but they are all pretty much the same...but let us know if you find a newer model that is really good.
 

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I'm not sure what they are called, but I call them 'jiggle pumps'. They start a siphon going with a slug or piston that is trapped inside the tubing, as you shake the tube, the piston jiggles up and down an draws the liquid from the tank into the tube. If you keep one end of the tube lower than the intake of the other end, it will flow all the liquid out.
Many boating, camping, and RV stores have them. Perhaps Wally World?
tom
 

Mark_88

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Yes, I had one of those...I called mine a Jiggle Hose...and I would walk around singing "I'm just a jiggle hose, and everywhere I goes..."

Gigolo...jiggle hose...lol
 

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UPdate:
Replaced: Fuel pump, Fuel filter, Fuel filler neck. IAC and Gasket
Truck is running like its 10 years younger.
Unfortunately the RPMS are still crashing periodically.

Friend of mine ran some tests and found that only half of each coil pack is firing when the RPMS crash.

So now we are leaning toward the Ignition Control Module perhaps being this issue. We saw that the top left bolt is missing from the ICM. So I'm wondering could this be a grounding issue with the ICM?
 


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