Engine Power Loss

**Israel Toledo** · 06-18-2012

I recently gave my 1998 Grand Prix GT, 6 cylinder, 3.8 litter engine a tune up involving plugs and wires. Additionally, did a diagnostic and codes for the MAP sensor and the (downstream) O2 sensor came up. I replaced them both. Gave it a motor oil and filter change. However, I noticed a reduction in RPM's. Car used to idle at 950 but now idles at 850. I did another diagnostic and no codes were present. "Check Engine" light reset and has remained off. I am at a loss as to the reason for the power loss. Some say it might be the catalytic converter. I welcome some advise.

**mechguy** · 06-18-2012

Can you describe the "power loss"?

As for the idle, yours sounds normal.

**Scottydoggs** · 06-18-2012

thats not a power loss, thats a rpm drop at idle, and 850 is where you want to be. i bet it used to be at 950 because of the sensor's being bad, now they are all communicating and its back where it belongs rpm wise.

**Fivefingerdeathpunch** · 06-18-2012

Hot in P/N is like 750 and in gear Hot is 685 IIRC

**Israel Toledo** · 06-18-2012

When I used to take the car to Pontiac, the mechanic explained that my engine idle should be at 950 as normal. Now that it's at 850 it's as though the car is going to shut off. There is a fluctuation on the RPM meter and you can also at times hear a very faint misfire while in idle. When you rev the engine the misfire becomes obvious. I just cleaned the Idle Air Control (IAC) valve which was covered in carbon buildup with alcohol as some suggested but the condition still persists. I'm curious, would the catalytic converter, or the Mass Air Flow (MAF) sensor be the culprit even though there is no code for it on the diagnostic? That's the one thing I did not change.

I apologize for the bad label on my problem. Scottydogs was right. It's not a power loss, it's an RMP drop. When on the road the car responds normally. When idle at a traffic light the engine runs a bit rough. The rough idle is more prevalent when you first start it. But while in (D)drive the idle is either at 900, or slightly above. I don't know if I'm explaining it correctly. I'm new at this.

**Fivefingerdeathpunch** · 06-19-2012

What brand of plugs?

**matt5112** · 06-19-2012

stock idle is 750 in PN, 685 in gear.

Vacuum leaks result in surging and other issues.

Try unplugging the MAF.

If it is the culprit the vehicle should behave much better with the MAF disconnected. It will know a DTC for it, but you know why its on.

**Israel Toledo** · 06-19-2012

Originally Posted by Fivefingerdeathpunch

What brand of plugs?

I purchased GM Iridium plugs at Abeloff Pontiac parts department. They told me they did not need to gapped. I also purchased wires from them.

**Israel Toledo** · 06-19-2012

Originally Posted by matt5112

stock idle is 750 in PN, 685 in gear.

Vacuum leaks result in surging and other issues.

Try unplugging the MAF.

If it is the culprit the vehicle should behave much better with the MAF disconnected. It will know a DTC for it, but you know why its on.

I will do that right now and let you know what happened. Thanks!

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