I had two cooling system incidents that have been resolved with advice from here and a new radiator, hoses, and thermostat.

After the repairs, engine speed has been 100 to 200 RPM faster than usual. With driving, idle speed seems to have resolved itself but, in gear, speed is still higher. For example, at 60 MPH in 5th gear, engine speed used to be 2100 RPM but it is now 2300 RPM.

What could have over-heated that would result in this ?

Thank you.

Engine speed difference at idle would have a different cause from a discrepancy between rpm vs mph. The only thing in common is the tach, or in other words the observation of engine speed difference, whether by rev counter or pitch memory.

Short of "utterly ridiculous" I'd say you are utterly discerning of your car's behavior. BTW there's a cable between your left foot and the clutch, no slaves.

I can't seriously guess at any relationship with work on the cooling system.

What a puzzler....
--
Art Benstein near Baltimore

The professor discovered that her theory of earthquakes was on shaky ground.

I don't think there is any connection with the cooling system work. The problem could be a result of overheating.

I don't suppose you've replace the speedometer at some point have you. The speedometer for a standard transmission is different from an automatic.

Just a thought,

Ron J

i think you're confusing the transmission with the differential.

it is the rear axle ratio which determines the signal sent to the speedometer not the transmission at least in the car being discussed here.,

since 5 speeds have 3.31 ratio axles and auto have 3.73 i can see a possibility of a difference but to my knowledge volvo installed the same speedometer in all sedans irrespective of the transmission and a different one in all wagons irrespective of the transmission.

Hi Art,

The others have commented on my original response about (remote possibility of) a slipping clutch (re: "... (unless your clutch is literally melting :-)....) so we'll leave that.

But I also offered that maybe, "...Only some electronic defect developing in your '93's speedometer circuit board might possibly explain this...." So, since you're the electronics guru on this forum, can you think of anything on that circuit board that could slightly change the indicated rpm? I have no idea how the circuit changes the pulse frequency from the rear axle into an analog speedometer reading -- but could some capacitor or resistor that's failing slightly change the reading? Something that would alter the calibration, if it's possible?

I once (for fun) "invented" a crude tach for my garden tractor -- it took pulses (via a resistor) from a wire wrapped around the spark plug lead and simply charged up a capacitor that drained through an analog meter and a resistor: the higher the pulse frequency, the higher the voltage in the capacitor and the greater the current flowing through the meter. It took a lot of adjustment of the resistor and some trial and error finding a suitable capacitor to come close to the right speed, and it wasn't a good correlation over the rpm range, but I had some fun doing it. Anything like that here, assuming real engineers could make it work better than I could :-) ?

[BTW, that was years before a cheap little digital/LCD tach for single cylinder engines started showing up in tractor parts counters. :-) But having grown up building HeathKit projects (anyone else old enough to remember those?), I still like to dabble. Too bad about Radio Shack, though -- they used to be useful -- but thankfully I've still got a real electronics component store near me to satisfy my urges :-).]

re: "...For example, at 60 MPH in 5th gear, engine speed used to be 2100 RPM but it is now 2300 RPM...."

5th gear? So it's a manual transmission (automatics only have 4 forward speeds)! That precludes a possibility of Torque Converter slippage because there is no TC. And that means that, when you're in gear, your engine crankshaft is firmly linked to your wheels' rotation (unless your clutch is literally melting :-).

So assuming that this is based on your instrument panel speedometer, there is no way that you can have a road speed:engine rpm ratio change as you described. And this is regardless of cooling system work (or even tire changes, since your displayed speed is based on wheel rotation, not tire circumference). Only some electronic defect developing in your '93's speedometer circuit board might possibly explain this.

If you're basing your speed on a GPS (Garmin, etc.) which tells you road speed, however, then there are factors involved that could indicate this change, including the occasionally awkward position of satellites (called the ephemeris), etc. But these would be temporary.
Also, a change in tire circumference might show up as a change in GPS-based road speed.

I would suggest that the precision (or lack of) with which you examine your speedometer display is likely the blame.

Could be the clutch is slipping? Cooling system repairs are not related.
Dan

Dan, how long can a clutch last with a 200 rpm slippage like that?

We let a clutch slip briefly when we start off, and maybe very briefly when we don't synchronize our throttle with our shifts sometimes :-); but if it's slipping continuously while just driving along, it can't last long, can it? I'd think he would have discovered clutch problems pretty quickly.

What do you think?

Yes, it is a manual transmission. Engine speed determined with in-dash tachometer and, at idle, by ear.

Hello,

A slipping clutch or maladjusted clutch slave cylinder could be more like it. Had owned a manual transmission car last time.

Regards,
Amarin.

I appreciate the clutch suggestions. (Although the theory doesn't cover high idle.) Could an overheated engine leak oil onto the clutch ?