overheating after switch to early heat exchangers

[shamrok]

Cerakoted heat exchangers from a 74. Dual-in, dual-out Dansk muffler. New backdated oil lines. No other changes and I'm hanging out between the top white line and red danger area on a ~55 degree ambient day.

Heat was previously backdated and heater hoses are all hooked up, no change there. Ignition advance and AFRs are same as before (EFI-managed). Fan belt is new and tight, oil level is good. I have a slightly crushed oil line from the external thermostat to the front cooler, but it's been like that since I got the car so I'm hesitant to start hemorrhaging money on new lines.

Before I start pulling out and testing thermostats, anything obvious I need to look at?

[Nditiz1]

I would start checking the sensors and the gauge AND check with a heat gun.

When I went from stock SC exhaust to SSI my temps actually improved.

[shamrok]

It's pretty clear it's overheating, performance just falls off a cliff around the top white mark, just bogging down and really sluggish. I suppose I'll look at the sensors, but she's not happy.

[kltarga72]

Originally Posted by Nditiz1

I would start checking the sensors and the gauge AND check with a heat gun.

When I went from stock SC exhaust to SSI my temps actually improved.

Plus 1.
I back dated with Dansk two in and one out. I also swapped the external thermostat.
A Lazer heat gun will be very helpful diagnosing oil cooling system.

[Discseven]

If engine temp spiked after installing the Cerakoted heat exchangers, and non-coated exchangers would not spike temps, am inclined to think the Cerakote is the possible cause. The coating could be acting as an insulator. Heat buildup in the exchangers is transferring to the engine---am speculating here. Quick way to get some intell on this is to call Cerakote tech and ask them about the insulating properties of the product on exhaust parts. If not the Cerakote, am inclined to think thermostat or temp sensor failure occurred coincidental to the exchanger install. Or there's something new sitting atop the oil cooler blocking air flow. If you have a friend with a similar 911, making temp comparisons with a gun might prove interesting.

[David Inc.]

You should try to fix that crushed oil line, but you shouldn't be overheating in those temperatures even without the front cooler.

What's the setup on your oxygen sensors for the EFI going from the old setup to the new? If it's the standard setup on the new exchangers with only one oxygen sensor then your sensor is seeing half the flow and half the heat load that it saw previously, so I think it would be prudent to re-tune your system, comparing oxygen sensor readings to results from an exhaust sniffer.

I'm going to quote Andrew Cologne's extremely helpful page on this:
https://nineelevenheaven-wordpress-com.translate.goog/the-lambda-control-of-the-911-sc/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp

"However, since the correct function of the original, unheated lambda sensor depends heavily on the exhaust gas temperature, the SSI system actually brings here two disadvantages:

- the probe sits further back towards the tailpipe, due to the design of the SSIs where the header is further back, this distance is already sufficient to cool the exhaust gas by a noticeable amount. Remember, the probe needs 500 – 800°C to work properly and a few more cm of travel means a too cold resulting probe.
- the volume of the exhaust stream is about half, compared to the original, where the exhaust channel of both heat exchangers are merged before the probe and the catalytic converter. This also results in a lower temperature at the probe compared to the original heat exchangers. With the SSI system, only the exhaust channel of one cylinder bank, ie the left one, is measured.

At normal speeds, this is not a problem because there is still enough exhaust gas temperature at the sensor to let it work correctly.
At idle, however, it looks different, here comes to the above 2 points in addition to the low engine speed, which allows even less exhaust gas volume and temperature at the sensor, whereby this cools down so far that the sensor reacts only very sluggishly , if not even fails."

[shamrok]

Definitely gonna zap things with my laser thermometer when this happens again and see if the thermostat is doing its thing and/or whether the Cerakote is actually providing an insulating effect.

Regarding the O2 sensor, and it seeing half the heat as before, that's an intriguing theory, but I'm not sure it applies here. It's a more modern LSU 4.9 sensor than the SC's lambda sensor and works immediately on power up, regardless of temperature, as the ECU uses its readings on startup to add warmup fuel. I can certainly retune, but if the readings are incorrect, it'd wouldn't be correct anyways? Will look into this some more, thanks for the suggestion.

The performance fall off is exacerbated when cruising at low load. If I stop at a light for a minute or so, things seem to cool down, which is counterintuitive to what I've ever experienced in this car, so I'm wondering if I am running lean and the ECU isn't picking it up. I need to compare temps on each bank and see if maybe I have an issue one bank (exhaust blockage? idk).

[911obgyn]

Make sure there are no leaks before the O2 sensor. Even small leaks will affect readings. I have used a shop vac to pressurize exaust system after spraying bubble solution on connections and found tiny leaks.