Strange intermittent "thump" within dash on '93

[RyPow]

My '93 is making a very odd sound that seems to be originating from the firewall behind the center of the dash. It is a "thump, thump, thump" at about one beat per second, 3-4 beats in a row then pauses for a couple seconds, and repeats. Literally sounds as if someone is banging their fist on the firewall behind the dash. It only occurs under these conditions:

- engine is warm
- light to no throttle
- when driving/moving (see below)

Vehicle speed does not seem to influence the thumping. There seems to be no effect on how the engine or any system (steering, trans etc) runs when the thumping starts up. I can feel it in the gas pedal, but not in the throttle cable under the hood. The noise always begins when driving, and when I come to a stop it gets fainter and fainter then disappears. It will not begin at a stop, and I cannot get it to happen by holding higher revs at a stop. When it gets going, increasing the throttle seems to stop the thump and it will start again when the throttle is released.

I suspected a blend/recirc vacuum actuator malfunction at first, but the sound occurs regardless of temperature or vent setting, or blower speed. Still doesn't rule out some sort of issue in that department.

I just wanted to see if there were any opinions or experiences with this, before I start tracing the condition. It is a real PITA since the sound gets fainter and quits soon after I stop!

[behlinla]

I'll be honest, I really have no idea what that could be. Do you have a zombie living under your car?

Does it still do it with HVAC completely turned off (fan and all)? Does the cadence of the thump ever change (with speed, temperature, RPMs, anything)? It's not the radio/speakers is it (operates normally)? Is the coolant bottle filled up and not leaking?

[pawpaw]

My first thoughts were the blend door acting out too, or a cooling system/heater circulation problem. Does your engine temp operate in the upper range of normal, or has its usual operating temp range recently changed????
So, to isolate the blend door, disconnect & plug the heater control under hood vacuum feed connection & see if the noise stops, if not then maybe begin a cooling system/heater circulation problem.

I've heard of Rangers of this vintage with the 3.0L Vulcan engine with "thumping" noise, caused by a scaled up engine block, causing hot spots that would cause the coolant to boil & cause a thump.

Also water pumps with eroded vanes from using the wrong coolant recipe that aerated & caused cavitation & that eroded the water pump vanes.

Or not changing the coolant on time, such that it became acidic, etched away at the water pump vanes, causing poor coolant circulation & that along with block scale, caused hot spots that would boil the coolant & cause thumping.

Some 3.0L Vulcan Rangers of this vintage didn't have the engine heater coolant bypass hose, that let coolant circulate freely from the water pump through the block, back to the pump while the engine was warming up before the thermostat opened, so adding the bypass hose increased coolant circulation in the block, reducing temps around the cyl water jacket, preventing the coolant boiling & causing thumping.

More thoughts for consideration keep us posted on your findings.

[RyPow]

Originally Posted by behlinla

Do you have a zombie living under your car?

The thought crossed my mind...

Originally Posted by behlinla

Does it still do it with HVAC completely turned off (fan and all)? Does the cadence of the thump ever change (with speed, temperature, RPMs, anything)? It's not the radio/speakers is it (operates normally)? Is the coolant bottle filled up and not leaking?

Yes, it does it with the HVAC turned off completely. The cadence of the thump will sometimes speed up erratically for a moment, then settle back to it's regular pace. As far as I can tell, it does not thump at higher revs or throttle. It is not the speakers, as the thump FEELS the strongest in the gas pedal and heater box, not in the doors at all.

Originally Posted by pawpaw

Does your engine temp operate in the upper range of normal, or has its usual operating temp range recently changed????
So, to isolate the blend door, disconnect & plug the heater control under hood vacuum feed connection & see if the noise stops, if not then maybe begin a cooling system/heater circulation problem.

I've heard of Rangers of this vintage with the 3.0L Vulcan engine with "thumping" noise, caused by a scaled up engine block, causing hot spots that would cause the coolant to boil & cause a thump.

Also water pumps with eroded vanes from using the wrong coolant recipe that aerated & caused cavitation & that eroded the water pump vanes.

The temperature gauge only ever gets slightly above half, and will go as low as 1/3 when the thermostat opens and the rad fan is on. This is at ambient temperatures from 15-40F (the only conditions I've driven the car in so far). I replaced the rad and water pump before I put the car on the road a month ago, and used the correct green coolant. No experience with temperature reading prior to then. When I changed the water pump, the passages in the block looked clean.

UPDATE: I discovered that the thump persists for about 10 seconds or so after the engine is turned OFF. It gets softer and softer until it disappears. This screams coolant or vacuum issue to me! Going to disconnect and plug the vacuum to the HVAC and see if the thump still occurs.

[Jeff K]

I think you might be on to something. Maybe a trapped air bubble in the cooling system that hasnt worked its way out??????

[pawpaw]

Well I'm sorta inclined at the moment to lean toward a system vacuum leak, say something like a blend door flapping, as the vacuum reservoir leaks off when the engine is shut down.
As suggested above, have you tried disconnecting & plugging the under hood dash hvac control panel vacuum feed line, to see if the thump stops???? If it does, look for a system vacuum leak.

If you think is a cooling system noise, you could do a "feel" test of the heater coolant hoses when its noisy, as if its coolant boiling & causing the thump, you should be able to feel it in the heater hose.

More thoughts for consideration, let us know what you find.

[RyPow]

Update on this issue:

The thump is definitely in the cooling system. The other day, it kept going when I pulled up to my shop. I felt the heater hoses immediately, and they are pulsing noticeably in time with the thump. Looks like a good lime scale flush is in my Vulcan's near future. The last thing I want to do is bust a hose or gasket by letting this issue persist. The car is running great otherwise.

[behlinla]

Yeah, it sounds like the coolant is bumping and causing the thumping. However it's very rare that you would have the right set of conditions to cause that to happen. Perhaps someone was using hard water in the coolant system at some point.

What does your coolant look like? Is it dirty?

Remember to use only distilled water to mix your coolant 50/50.

[RyPow]

Originally Posted by behlinla

Perhaps someone was using hard water in the coolant system at some point.

What does your coolant look like? Is it dirty?

Remember to use only distilled water to mix your coolant 50/50.

I replaced the water pump and radiator when I got the car in December and refilled the coolant. I only ever use distilled water to mix coolant. The old stuff wasn't very dirty from what I recall. I've only driven about 2000 miles since the water pump job and the coolant still looks fresh.

I wouldn't be surprised if hard water was used at some point in the past. Some people like to cut corners

[pawpaw]

does your 3.0L have the heater bypass hose & is it in good shape & flowing freely????
Are you using a Motorcraft, or like design thermostat that has the bult in air bleed valve & its of the proper heat range???? If not, are you certain you got a good system air burp???