I also tackled the temp gauge mismatch issue. I had a theory about what was causing the abnormally high temp indication. The gauge was showing 235-240 while the ECU was seeing around 220.

Here is what I thought was going on. There are only two places on an LS to normally mount a temp sender. The “normal, factory” sender location is in the left cylinder head near the front of the engine. Since the heads will work on either side, the other location is in the rear of the right head (left head flipped around). The Speedhut sender uses 1/8” pipe threads and the LS head has straight metric threads, so I ordered a brass adapter from Speedhut when I ordered the gauges. On my car the firewall wraps around the engine pretty tightly and is insulated on the inside of the passenger compartment. To make matters worse, there is a nice heavy steel exhaust manifold runner right by the sender.

On the highway, there is enough air flow through the engine bay to keep everything reasonably cool, but in traffic, the heat from the exhaust was being reflected back to the large brass adapter and brass sender. All this junk was getting heat soaked. I was reading some exhaust temp and water temp. Due to it’s front location near the exit to the radiator, I felt that the front ECU sender was actually seeing what should be the hottest water in the engine.

I got lucky in that Holley had provided a hole threaded for 1/8” pipe on the back of the water pump right in the center of the return to the radiator. This was normally for a steam line fitting from the heads, but I ran my steam line to the expansion tank. I ordered a new sender (no way to get the old one out. Too close to the firewall) and a new harness. I spliced this to the old harness and moved the sender to the water pump. Basically, the same deal as putting the sender on the intake manifold of a Gen 1 SBC.

This worked pretty much perfectly. Up to 200, the gauge reads about 5 degrees colder than the ECU, but after that, they match within 1-2 degrees. As far as I’m concerned, that problem is solved.



If you have been following this build since the beginning, you may find it ironic that on my maiden voyage home from Florida 8 years ago, I had false overheating problems due to a misplaced temp sender and I had nearly the same issue on this maiden voyage. Fate? Irony? Coincidence? Maybe all three.

Worked on an electrical issue that was bugging me. The voltage seemed to be all over the place. It would read 12.5 and then go to almost 14. The dash gauge and ECU agreed, so I figured it was a real issue.

The Holley accessory drive I’m using uses an alternator that appears to be based on a very late model Corvette part. May even be the same part. It is rated at 150 amps (I think that’s right) and is a 6-phase alternator. Basically, it’s two 75 amp 3-phase alternators in one case, which is supposed to allow it to put out more amperage at low RPM. A little research turned up that the Corvette units also have a more sophisticated voltage regulator than older GM alternators, that can supposedly analyze the battery condition and has a wider range of outputs to correct issues and save fuel. Couldn’t save much fuel, but I guess every little bit helps. 12.5 to 15 volts is normal and Corvette owners reported that the voltage would vary in that range at any given time.

I was about to dismiss my issue as a non-issue when I noticed the voltage falling to 12 on the ECU. Gauge concurred. Nope, that ain’t right. I checked the alternator output and it was at 14.7 while the fuse block was showing 11.7! I found a loose connection on the battery disconnect. After tightening it up, the voltage settled out at 13.5.