500 RPM dip and rumble feeling from engine. No Codes.

[TonyG1989]

I'll drive for a while, and come to a red light, in drive with brake engaged it will "every once in a while" dip down to 500 RPM and the weirdest rumbling comes from the engine. Has anybody seen this before? AC coming on and the slight dip it makes produces almost the exact same rumble, identical as a matter of fact. It will still do this IAC unplugged and with the AC off. When I give it some gas and bring the RPM over 1000 she goes back to normal and will act right indefinitely. I would just wait for it to get worse but it sounds hard on the engine? Something isnt happy in there.

Im going to try and reproduce the symptom in park. I'll report back. I can't get it to do that rumble stuff in park but ill keep trying, I was thinking maybe torque converter? The 3.0 uses a harmonic balancer so it couldn't be that, right?

 

[franklin2]

Is it always after it's been running awhile and in town? What type of cooling fan do you have? Could it be the cooling fan kicking in, whether electric or mechanical with a clutch?

 

[TonyG1989]

Yes has to be driven for a while. Just a clutch cooling fan. I don't know for sure, it could be honestly.. But I have started suspecting the torque converter. Could it be the TCC Solenoid going out?

 

[franklin2]

I thought when they went out it would stall at a stoplight. I have had the torque convertor clutches shudder, but it only does it usually pulling a hill in high gear.

 

[TonyG1989]

Theres a weird vibration at 3000 RPMS, and when it does this 500RPM drop randomly, it is always at a stop light.

 

[RonD]

I would clean IAC Valve, and the rumbling could be power steering pump at lower lower RPMs, as said like with AC on

 

[TonyG1989]

I installed a brand new IAC today just to make sure. Same issue. Power steering pump. Hmmm.

 

[RonD]

The computer is very reactive to idle RPMs when your foot is off the gas pedal
The IAC valve/computer combo is accurate to +/- 4 RPM, and it reacts very fast, under 1 second
To the computer this is just Math, TPS under 1volt + Voltage at IAC Valve = XXX RPMs

Assuming automatic, try shifting to Neutral as you approach a stop, that will tell you if is a torque converter issue

 

[TonyG1989]

The computer is very reactive to idle RPMs when your foot is off the gas pedal
The IAC valve/computer combo is accurate to +/- 4 RPM, and it reacts very fast, under 1 second
To the computer this is just Math, TPS under 1volt + Voltage at IAC Valve = XXX RPMs

Assuming automatic, try shifting to Neutral as you approach a stop, that will tell you if is a torque converter issue

When it starts doing this weird 500 rpm idle thing, and I put in in neutral, it goes away. But ill try to see if I can make it happen in neutral now.

GREAT STUFF, thanks for the knowledge there, I may be changing my TPS next based upon this.

 

[RonD]

Just test the voltage at the TPS, center wire, use a sewing needle to pierce the wires insulation
Key on engine off
0.69 to 0.99v is expected at closed throttle, 4.5v wide open

Or if you have an OBD2 reader call up TPS
16 to 19% throttle close
90-93% wide open

Its a 5volt sensor, 0 to 5volts, so 1 volt = 20%, under 1volt 19% of 5volts
Wide open 4.5v = 90% of 5volts

Just thought I would throw that in because people often ask why its not 0% with throttle closed and 100% at wide open, that's why

 

[TonyG1989]

Just test the voltage at the TPS, center wire, use a sewing needle to pierce the wires insulation
Key on engine off
0.69 to 0.99v is expected at closed throttle, 4.5v wide open

Or if you have an OBD2 reader call up TPS
16 to 19% throttle close
90-93% wide open

Its a 5volt sensor, 0 to 5volts, so 1 volt = 20%, under 1volt 19% of 5volts
Wide open 4.5v = 90% of 5volts

Just thought I would throw that in because people often ask why its not 0% with throttle closed and 100% at wide open, that's why

It shows 20% percent on the OBD2 scan. It seems to function properly. Could it be getting hot and going on the fritz occasionally?

Also, is a loose timing chain a possible culprit?

 

[TonyG1989]

Just test the voltage at the TPS, center wire, use a sewing needle to pierce the wires insulation
Key on engine off
0.69 to 0.99v is expected at closed throttle, 4.5v wide open

Or if you have an OBD2 reader call up TPS
16 to 19% throttle close
90-93% wide open

Its a 5volt sensor, 0 to 5volts, so 1 volt = 20%, under 1volt 19% of 5volts
Wide open 4.5v = 90% of 5volts

Just thought I would throw that in because people often ask why its not 0% with throttle closed and 100% at wide open, that's why

Just test the voltage at the TPS, center wire, use a sewing needle to pierce the wires insulation
Key on engine off
0.69 to 0.99v is expected at closed throttle, 4.5v wide open

Or if you have an OBD2 reader call up TPS
16 to 19% throttle close
90-93% wide open

Its a 5volt sensor, 0 to 5volts, so 1 volt = 20%, under 1volt 19% of 5volts
Wide open 4.5v = 90% of 5volts

Just thought I would throw that in because people often ask why its not 0% with throttle closed and 100% at wide open, that's why

I remember you saying that in another thread. That the values are linear or something.

 

[franklin2]

Sometimes you have to just keep driving it. If it gets worse, then the cause is usually more apparent. If it stays the same, then it's probably just some minor adjustment somewhere. You are aware of it, and are doing due diligence. I certainly would not spend a lot of money on a guess, you probably have already spent plenty of money on it. Of course you now know what it isn't.

 

[rusty ol ranger]

When it starts doing this weird 500 rpm idle thing, and I put in in neutral, it goes away. But ill try to see if I can make it happen in neutral now.

GREAT STUFF, thanks for the knowledge there, I may be changing my TPS next based upon this.

If it goes away when you hit neutral you have a trans issue. The engine load in drive at a light and neutral is almost zero anyways.

I had a 94 Econoline that would do something similar...i wrote it off as an IAC and since it was a van and a bitch to change id run it till it totally quit.

The O/D light started blinking and i dont remember the codes, but it was the lockup solenoid i believe...along with a speed sensor. Eventually it got to the point youd put it in gear it would just stall.