Carpokes Porsche Forum

Thanks to playing with a standalone ECU, I've been able to read the DME temp sensor directly and learned my car has been operating at well over 110C, typically as high as 120C. The gauge sensor reads low, so I never knew, but it explains why I was often getting coolant steam despite confirmed multiple times I had no leaks. At first I thought the ECU was programmed wrong, but confirmed the right pull-up resistor and sensor settings, and verified the coolant temp with a infrared thermometer aimed at the coolant crossover pipe.

My first thought was thermostat, so I went through that (painful) replacement, and replaced the aux turbo thermostat, and DME temp sensor while I was at it, as well as a fresh rad-to-pump hose. I meant to replace the gauge temp sensor as well, but somehow ordered a two prong sensor. I got the same behavior afterwords, so asked Gemini for troubleshooting help and it suggested the water pump, as indicated by feeling pressure surge in the radiator hose under revving, or seeing flow into the reservoir. I can't figure out how to reach the hose while revving, so just viewed the reservoir with the cap off - I seem to get intermittent flow, and not a whole lot.

I'm guessing that means the internal pump impeller is slipping on the shaft? But it's a big expensive job to replace, so wanted to double check any other culprits while I'm at it. Is there anything else it can be? Are there other steps I can do to verify a faulty pump? Not getting any bad sounds or leaks from the pump, although I did notice a very loose fit for the internal thermostat seal that is supposedly hard to remove. Mine literally fell out with the thermstat removed.

Side question - is it possible to get very different temp readings at the DME vs gauge temp sensors? Just wondering if the gauge sensor is actually somehow outside the failed pump circuit so not getting hot coolant.

When was the pump last replaced? If the impeller is slipping on the shaft, that could be a cause. Do you think there is any air in the system, and at higher revs, could that cause cavitation....just thinking out loud here.....sorry I don't have a definitive answer.

ROB III wrote: Mon Jun 01, 2026 3:06 pm When was the pump last replaced? If the impeller is slipping on the shaft, that could be a cause. Do you think there is any air in the system, and at higher revs, could that cause cavitation....just thinking out loud here.....sorry I don't have a definitive answer.

I think the system is very well bled - both with a pump in the shop, and then purged again after operating. Given the heating issue, I wasn't operating it above 3k, maybe 3.5k - with interior heat on full blast, just to limp home.

Pump was last replaced shortly after I first acquired the car in 2011(!!!) and about 20k miles ago, and now that I check my notes a rebuilt pump was used by the mechanic. So I guess it's time to tackle this one anyway.

But I'll facepalm pretty hard if I do the replacement and my car still overheats

I would want to test both sensors first to get full confidence in their readings. We have test procedures for both in the 944 handbook. The two sensors are in different pockets of coolant but unless something is blocked they should see very similar temps. If the gauge sensor says it's not overheating, yet your think it is, I'd test both the gauge and the sensor as shown in the handbook. Last thing you want to do is pull it all apart based on a mistaken belief it's overheating if it really isn't.

Assuming it really is overheating, I wouldn't be too quick to conclude the impeller is spinning on the pulley. It's a possibility -- and there was a batch of water pump years ago that did have that problem -- but it's pretty far down the list of likely overheating causes these days. Other possibilities that come to mind -- aged-out radiator, bad fan operation, bad bypass seal behind the thermostat, air in the system, leaking head gaskets, debris in radiator fins, bent fins, too lean, too much ignition advance, etc. etc.

What are your symptoms? Is it always hot -- highway, stop and go, and idling in drive way? Does coolant spit up after a heavy drive?

Tom wrote: Mon Jun 01, 2026 8:12 pm What are your symptoms? Is it always hot -- highway, stop and go, and idling in drive way? Does coolant spit up after a heavy drive?

Thanks for the tips. You're right, I should definitely go through a full test of that remaining temp sensor and the wiring before I get further. edit - and I'm realizing I've been looking at the wrong part - the thermoswitch is the single pin sensor I've been looking at, not the two pin gauge sender. You'd think I'd remember that since I replaced it last year. Ugh, I guess this means I've got to tear things down again to get to the sender.

Symptoms are:
- Car is always hot. Heats up to 110C-120C whether idling or operating. Even cruising at 3000rpm with heat on, temp stays steady at 115-120. I have not operated the car hard to see if it gets better or worse with heavy throttle and high RPMs.
- I did get coolant boiling over into the engine compartment on this last test drive out, but some of that may have been overfilling the reservoir. Prior to that, I would get some coolant steam from underhood (which I was always confused by given normal gauge readings), but don't remember seeing coolant splattered around underhood like this last run. Notably, I didn't get overflow on my return drive to the shop.
- Fans seem to operate correctly, and continue to run during the turbo cooldown. I'm running the F9 SS fan relay (and DME relay), and recently changed the thermoswitch. And I get overheating at speeds where the fan should be irrelevant.
- Temp readings are via the DME temp sensor viewed through TunerStudio dash. They are inconsistent with the temp gauge as mentioned, which reads low, but verified with an infrared thermometer on the coolant crossover which read 115C shortly after shutting the car off. Also check at 80C and they were within 5C of each other.
- Coolant and oil are both completely clean.

What I've done:
- Prior relevant work: replaced aforementioned thermoswitch and cooling fan sensor, bled coolant system, replaced fan relay (F9 SS)
- I assumed a bad or blocked thermostat, so I replaced the thermostat (w/ new late gasket, washer, and snap ring). Found the inner thermostat seal floating in there loosely, so thought I'd found the culprit for a blockage. Replaced inner seal and noted a loose fit, rather than the press fit it should have (see pick below confirming placement, but it was looser then a slip fit. Don't worry about coolant color - that's UV dye to help chase down leaks).
- While I was in there, I replaced the aux/turbo thermostat and o-ring, DME temp sensor, a few hoses, and the coolant reservoir.
- Bled system with pump first, then after warmup in shop, and then again after test drive. I did get a bit of air even on the last bleed. I'll get back to the shop this week for some more testing, but if I had to guess about the gauge sensor it's either a bad sensor or bad wiring. Edit: it's also possible that I damaged the gauge cluster when i recently refurbed it, as I do remember the gauge functioning properly a few years ago when there was some air in the system that would allow intermittent overheating.

Guessing about the overheating, it feels like either a blockage or pump failure. Not sure how to track down a blockage...

to answer your prior question about the gauge temp sender, I suppose it is possible for it to be way off, but I haven't really heard about that. Recommendation if you suspect that is to clean the connection at the sender and at the wire terminal.

Which standalone system? You also have to make sure your sensors if not matched to a predefined option are scaled/calibrated to read correctly.

AudiSport wrote: Tue Jun 02, 2026 6:44 am Which standalone system? You also have to make sure your sensors if not matched to a predefined option are scaled/calibrated to read correctly.

Been developing a Speeduino based ECU with classicecu.com. I checked with him to confirm that the board resistor is the 2490 Ohm that I have in the settings, but I believe it's correct based on checking it against the infrared thermometer. I also confirmed the resistance curve on the Bosch sensor (which is listed as the "BMW 325i" in TunerStudio, but can also be manually entered).

I'd strongly suggest you test the gauge sensor as shown below. This is the last page in out guide and the most accurate way to test it. If the sensor tests ok, then next we can show you how to test the accuracy of the gauge in the car. If they both check out, then at least you can have confidence in the actual temps and are not chasing ghosts. Based on your various comments, at least from this side of the internet screen, it seems like there is still uncertainty about your real temps. One the other hand, spitting up coolant and seeing steam is never good -- so I'd also want to pressure test your cooling system to test for any leaks, and look in the oil cap for any signs of mocha sludge.

Have you checked actual temperatures at the sender, radiator inlet/out using an inrfrared temperature gun? These can be had for cheap---the inexpensive ones have limitations and one needs to be aware that different materials and colors can emit varying ranges in the IR spectrum, but one can establish a general trend by carefully tracing the cooling system according to the diagram in the link below.

http://www.clarks-garage.com/shop-manual/cool-10.htm

I have seen at my shop a couple of instances of slipping water pump impellers in 944/968 circa 20 years ago, we had a reliable rebuilder for a number of years, but that became less so . . . . have used only new pumps ever since (Laso and Geba brands have been good). But I would first be sure that the cooling system is truly bled. Is there hot from the dash vents with the heater valve open?

The advanced move would be an IR camera or attachment for your smartphone, I use this one several times a week at my shop:

https://www.oasisscientific.com/collect ... windows-pc

In addition to quickly establishing if there is cooling system flow, it can be used to detect a misfire on an engine with accessible exhaust manifold/headers (misfiring cylinder will have a much cooler exhaust port/runner), HVAC systems, and can also detect battery drains if the vehicle has been resting in a cool-ish shop. Check out the pic below of a '76 Porsche 930 fuse panel, I forgot to adjust the manual focus, but it shows how hot the single ceramic 25A fuel pump fuse contacts get from running both fuel pumps (can also see the round fuel pump relays above). To prevent forest fires and decrease voltage drop in the fuel pump circuit, I rewire this circuit to bypass the old fuse block and use heavy-duty inline blade fuse holders straight from the battery positive terminal to each of the fuel pump relay terminal 30 wires: