Piston Slap: May The Best Car Lose, Mr. Lutz: Mehta Challenges Cadillac's HT4100

Yours truly, Sajeev Mehta writes:

Hello Piston Slappers, this S.O.S. is for anyone GM-savvy enough to tango with a 4.1L Cadillac. And live to diagnose another day: are you up for the challenge?

I acquired a 1986 Fleetwood 75 Formal Limousine (1 of 1000), which made me reconsider my stance on wrong-wheel drive GM products of the mid 1980s This was a $0.99 eBay purchase from a frustrated shipping company in the Houston Ship Channel who lost their overseas buyer. It didn’t run, until we installed a $20 ignition module. Which brought the less-than-a-buck Caddy back to life. Almost. So what’s the problem?

Tech Overload Warning! After fresh oil, coolant, belt, filters, vacuum lines, T-stat and a successful compression test, the trouble-prone HT4100 (at 84,000 miles) cannot idle below 1200 rpm, is hard to start, stalls going into gear and generates an E30 (ISC RPM Out Of Range) code. The ISC motor works and seems to be adjusted correctly. The TPS sensor is fine, according to the (ingenious) self-test on the dashboard. But these items needed replacement: coil, radiator, temperature sensor and alternator. While currently under $500 of capital invested, I’m now officially weary of throwing parts at this

So I am leaning toward addressing the Internet’s two biggest beefs with this Caddy: intake manifold gaskets (is there a horrendous vacuum leak?) or a worn distributor gear. There is some slack in the rotor, but those intake gaskets might be on their way out. Any guesses on resolving my problem? How offbeat is my latest diagnosis?

(Don’t waste bandwidth telling me to scrap the car: this Fleetwood has Farago-worthy levels of Cadillac brand equity in its copious hindquarters. Even the staffers at the 24 Hours of LeMons are in love. Chat with Autoblog’s Jonny Lieberman and Jalopnik’s Murilee Martin if you don’t believe me. More on that later.)

[Send your technical queries to mehta@ttac.com]