Driving at a steady 55 mph or 65 mph my '99 Eldorado does this surging thing, similar to a person pumping the throttle up and down.
No stored codes, no check or service engine light. Makes cruising speed challenging.
Car has had plugs and an IAC and plus a recent fuel pressure regulator replaced.
Throttle body was serviced, exhaust was replaced, no smoke, no steam, operates at normal engine temperatures.
It runs flawlessly. Only suffers from some mid case and oil pan leaks.
At 15 years old and 125,000 mile I feel this Caddy gives me all she's got but the surge at steady hi speed cruise speed is a little puzzling and annoying.

Any clues or thoughts?

 

No telltale codes popped up on me either.

 

Vacuum leak first. Cruise servo is very possible cause

 

You could also check the Idle Speed Controller (ISC). They may have checked the throttle body, but if the ISC is intermittently closing the choke, it could cause some surging. My last car was a 1989 Cadillac Sedan Deville, and it surged frequently - until we cleaned it out. That took care of it.

TS

 

The ISC was changed last summer. That repair solved my intermittent high idle problem.
Someone suggested a few days ago to swap out the TPS. Make sense to you?
Was checking vacuum systems yesterday but haven't found any leaks yet in the upper engine compartment. I'll search more elsewhere tomorrow.