Thump thump thump..

[Macneil87]

I have a 98 ranger 4L 5 speed. Once the engine warms up and only at idle, I hear a thump thump thump noise. Sometimes it’s clear as day, others it’s more quiet. I haven a clue which side it’s coming from as it sounds like it comes from both sides.

All functions work as they should; even the cruise control. I know these trucks are all essentially operated by vacuum which leads me to believe it’s something of these sorts but I wanted to ask if anyone else has run into this issue in the past?

I am an automotive mechanic, however just haven’t had the chance to look into in great detail as of yet. I’ve only had the truck about a month and has done it since the day I got it.

Thanks!


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[RonD]

I assume the frequency of the noise changes with RPMs?

So you believe it is an "engine noise"

The "thump" describes a low tone vs a "tick" which describes a higher pitch, like a rocker, lifter or fuel injector "tick"

And since it only comes after oil is thinner(warmed up) you may have a main bearing or rod bearing issue.

4.0l OHV engines are not known to have bearing issue but since the truck is new to you there is no history.

You can test for Rod knock by disabling spark on one cylinder at a time, if "thump" stops then that cylinder has the problem

Its possible it could be an exhaust leak
Or fan belt pulley noise

 

[Macneil87]

I forgot to add that the thumping will sometimes continue with the engine off. It’ll only thump/thud for 10 or so seconds. This is what leads me to believe it’s something to do with the vacuum system.


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[RonD]

Yes, that would be an important part, lol.

Not sure what vacuum operated system could cause a thump noise, there are only a few vacuum systems
EGR
EVAP
Cab Vents
Power brakes
PCV
PVH(4x4 only)


But a partially blocked heater core or cooling system passage can
If circulation is slow in an area of the head the coolant expands rapidly as it is overheated and can make that thump noise, and it can continue after shut down.
And would only occur after warm up.

A vacuum noise should not depend on engine temp
Although EGR system is only used after warm up
EVAP maybe but it is usually used at startup

There are 3 vacuum "motors" in the cab that open and close vent doors, these doors can make a thump sound, would be a first for me if they are causing the noise

 

[Macneil87]

I am leaning to the side of the noise coming from under the hood. I thought maybe the EGR also.

I just did head gaskets on it when I bought it. Cooling system is A OK


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