1984 Sedan DeVille with the ever so troublesome HT4100. My Service Now and Coolant Temp lights come on simultaneously as I am driving down the road, usually cruising at 35mph. Flash, and off again, sometimes a few times in a mile or so. Other times, just the Service Now light will show, car will stumble, and on my way again. ECC panel diagnosis will show a 52 - memory reset code,the first time showed a20 - open fuel pump circuit code, but all was fine. All parameters show OK in .9.0 checks. The distributor has a pickup coil that is slowly going badin it (intermittent no start when allowed to heat soak), but this should have no bearing on what is happening now.

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Code 20 check oil pressure switch connector, tend to deteriate. If service now light code should be store. If code 52 keeps coming back after being cleared, ecm is resetting. Ecm maybe bad or you may have poor ground. Original air & coolant temp sensor connector tended to fall apart also.

 

Checked the coolant temp sensor yesterday and it's barely holding together. I just gotta keep it running until the new engine is done, but that's been put on hold for about 6 months. Leaks all over the oil pressure sender, so that may be the problem. It shorted this past June and blew the fuel pump fuse, so I put on a spare I had off another HT4100 and it's been fine until now.

 

You have my symphaties. I had a 84 Eldorado with the digital dash and you cannot trust any gage on it. It would read 4 gallons and be on empty, and the 4100 engines came with bad cams.
Every cam had a weak lobe on it, so after 30,000 or so miles they develope an engine miss. I don't think cadillac intended to get it right on those 4100s.
I think they were thinking give the cars a short life span and sell more. They gave minimum warranties on those cars.