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Surge in 1999 Ford Ranger

When I turn on dial for fan, engine will surge every minute, will also do the same when air conditioner or heater is turned on. Dealer wants to inject colored flud in air cond line to find problem. Engine runs smooth when dial is in off position. Any suggestion befor I bring into Dealer, TX

They are suggesting adding dye to the A/C refrigerant? That’s completely irrelevant. You’d do that to help find a leak in the A/C system. A leak in the A/C system isn’t going to cause your engine to surge.

I don’t know the Ranger system for vent controls, but on many cars the vents are controlled by tapping into engine vacuum. It sounds to me like you just have a disconnected or otherwise damaged vacuum line for your vent controls. This would amount to a vacuum leak for the engine, and vacuum leaks often show themselves by a surging and falling idle.

But you also said almost nothing about the truck, its history, or this problem. It always surges, or is this an at idle thing? Do you have a tachometer? If so, what kind of surge are we talking about in terms of rpm rise/fall? How many miles are on the truck? How old are the plugs & wires? Filters? Has anyone checked and cleaned your battery & alternator connections? (The blower fan is an electrical draw - a power problem, such as a poor ground can affect how the engine runs).

You apparently took it to a dealer already. What did they do? Why do they want to add dye to the A/C? What was the explanation?

Why are you taking a 14 yr old truck to a dealer?

Truck has 150K just had 150K matiance done. Replaced alternator. Surge happnes every 10seconds, slight increase in rpm, plug and wires replaced last year, noticed terminals need to be cleaned. I may have miss understood service manager, dye may be added to vaccum line.

Is it only at idle and mostly goes away if you accelerate? That’s how a small vacuum leak would work.

A smoke test is what would be used to find a vacuum leak. But in this case since it is connected to using the HVAC system it seems to me that a basic inspection of the lines would be sufficient.

No, you wouldn’t add dye to a vacuum line. They might use a smoke machine to find vacuum leaks.