RPM FLUCUATION

[teresav]

I have a problem with the rpms going up and down in my 2004 with only 55,000 miles. The rpms surge about 100-300 only when I try to keep the vehicle coasting at the same speed. If I accelerate or de-accelerate, the rpms are stable. It idles fine. I had the transmission fluid and filter changed at 35,000 miles. I put in a bottle of lucas conditioner when this started to happen and there was no difference. The up and down rpm surge does not allways happen, but when it does, it is consistant with its timing. I mean that the surge just goes up around 200 and then back down 200, then up, then down, until I accelerate or take my foot off of the gas. The surge difference is only in the speed. If I drive 35 then the surge may be 100. If I drive 55 then the surge is 200, ect. I can slightly hear a diffence in the sound of the vehicle but with the tires and road noise its hard to hear. If I didn't have a gauge I probably wouldn't notice. No check engine light. I know that to diagnose it could be a variety of problems, but the consistant timing of up, down, I thought maybe someone could give me a clue of what to check.....or tell my mechanic to check.

[HenryJ]

That happened to mine when I tried the Bosch E3 plugs. Many others have had ignition problems after installing other plugs too.

The stock plugs were pretty good. They would be a suitable replacement. I like the Autolite XP Iridium plugs best. Those are the only two that will not give you problems.
It may be time to install a good set of plug wires. Taylor SpiroPro are less expensive than stock and a much better wire.

Open the hood at night with the underhood light disabled, if you installed one. With the engine running look at the light show and note areas where sparks are leaking. Isolate and adjust wires as needed. The side of the ignition coil is common. Ash will build up there usually.
Clean the engine and apply a liberal coat of cheap hairspray to the cap , coil and wires. This creates a static proof seal and aids in preventing crossfires.

The Autolite XP plugs and Taylor wires together will be under $100 and well worth the investment.

[teresav]

Thanks HJ. I have never replaced the plugs, let alone the wires. The truck is in the mechanics care right now due to some leaky oil lines and a leaking rusted power steering pump. If he doesn't come up with a diagnosis, I can have him change those too. Do you think that they would be bad after only 55,000 miles? I think that I remember them being rated for 100,000 miles.

[HenryJ]

Plugs are rated for 100k, but degrade over that time. Wires degrade over time too.
Those would be worth replacement, at the very least inspection.

Moisture fouling of the cap and rotor are notorious. Be very careful with the plastic distributor. Installing screws with locking compound with break the retention points! That will require distributor replacement Easy mistake to make. Don't make that a new problem.


I would suggest that a careful inspection of the ignition system is in order. If it has been a while, a new fuel filter and dose of injector cleaner might be a good idea too. The fuel pump needs a good clean and lube once in a while.

The throttle body can also carbon up and cause issues. It is pretty easy to open it up and swab it out with a rag soaked in cleaner. While that is apart clean up the mass airflow sensor too. A can of MAFS cleaner works well. It can also be done with swabs and some rubbing alcohol, but take care not to damage the tiny sensors.

Here's hoping it is as simple as a dirty filter