Surging idle and stumbles when revving

[RamblinDMC]

Hello all,

Now that I've resolved my coolant issues. I need to get my engine to idle and rev normally. I'm aware that the idle surging issue is common with DeLoreans, I'm a bit unsure about how to fix that. It also stumbles/sputters and sometimes backfires when revved. I can't seem to be able to drive it any faster than 20 mph. (Its an auto) The throttle progression gets to a certain point and, it just stumbles.
I did check the FV and I think its buzzing. If I unplug it, the engine doesn't change speed. I also tried unplugging the O2 sensor. (That's the plug on the driver's side firewall right?) The engine immediately dies when doing so.

Any ideas?

[Jonathan]

Hello all,

Now that I've resolved my coolant issues. I need to get my engine to idle and rev normally. I'm aware that the idle surging issue is common with DeLoreans, I'm a bit unsure about how to fix that. It also stumbles/sputters and sometimes backfires when revved. I can't seem to be able to drive it any faster than 20 mph. (Its an auto) The throttle progression gets to a certain point and, it just stumbles.
I did check the FV and I think its buzzing. If I unplug it, the engine doesn't change speed. I also tried unplugging the O2 sensor. (That's the plug on the driver's side firewall right?) The engine immediately dies when doing so.

Any ideas?

No, that's a different plug. It is confusing because of the blue wire, but it is not for the O2 sensor. The quick connect for the O2 sensor you reach from inside the wheel well (awkwardly at that).

What does your engine tachometer do at this stumbling 20 mph? Does your engine rev up to 5,000 or 6,000 when at 20 mph? Do you think it is staying in 1st gear? I only ask to see about ruling out a shift computer problem.

[RamblinDMC]

Oh that's right, the O2 sensor goes through that rubber gasket into the compartment behind the driver's side seat.
The engine RPM just kinda flat lines around 2500/3000 or there about. It just doesn't want to rev any higher than that.

[SS Spoiler]

Slipped throttle cable?

[David T]

Hello all,

Now that I've resolved my coolant issues. I need to get my engine to idle and rev normally. I'm aware that the idle surging issue is common with DeLoreans, I'm a bit unsure about how to fix that. It also stumbles/sputters and sometimes backfires when revved. I can't seem to be able to drive it any faster than 20 mph. (Its an auto) The throttle progression gets to a certain point and, it just stumbles.
I did check the FV and I think its buzzing. If I unplug it, the engine doesn't change speed. I also tried unplugging the O2 sensor. (That's the plug on the driver's side firewall right?) The engine immediately dies when doing so.

Any ideas?

Could be a lot of reasons beginning with vacuum leaks, old spark plug wires, worn spark plugs, dirty fuel injectors, etc. You really have to eliminate or fix each possibility one-by-one. A properly running Delorean should not hunt much, the better it runs, the less it will hunt.

[RamblinDMC]

Turns out it was a combination of the fuel mixture screw not adjusted correctly and the vacuum hoses going to the spark advance, vacuum solenoid, and the manifold tapping were not connected. It still stumbles if you rev it quickly. You can rev it slowly and it will run just fine. Things such as putting it in gear causes it to die.

[Jonathan]

A local owner near me with an auto had somewhat similar problems and they were related to the vacuum advance solenoid. Sounds like you might want to take the time to go over all the vacuum connections and routing and make sure everything is correct. And check your fuses too. This owner I mentioned had the fuse keep popping associated with the vacuum advance solenoid. See if yours are all the correct rating and are still intact.

[RamblinDMC]

I'll check into the vacuum advance solenoid. Thanks!