Evasive Rough Idle

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Aug 1, 2004 | 07:31 PM

autotech

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Racer

Joined: Oct 2003

Posts: 273

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From: Midland Texas

Evasive Rough Idle

88 - C4 - L98 - 5.7 - Mild Cam & Flat Tops

I built a new motor and it just idles a little rough. Here is a list of what I have replaced for other reasons and for idle quality.

ECM
IAC
Pick-up in the dist.
cap and rotor
ignition module
plugs and wires of course

I have not replaced the TPS. I have no codes. No apparent vacuum leaks. The MAF shows to be good, (I cant make it faulter by tapping on it). I did replace the injectors with some SLP's, (the cheap ones). Fuel pressure is good. Everything is extremely clean, ie: throttle body, IAC. It will idle a little low, say 500 rpm. I wonder if I got a bad IAC, but it falls within the voltage specs. required. I'm lost, and I do this for a living, lol... Any input appriciated...

Aug 1, 2004 | 07:47 PM

CFI-EFI

Race Director

Joined: Sep 2000

Posts: 17,298

Likes: 33

From: The Top of Utah

Is the cam really that mild? Have you tried playing with the minimum idle? How about the injectors? Are they the same size? How is the idle A/F ratio? Does more base timing help? Just a few things to look into.

RACE ON!!!

Aug 1, 2004 | 07:49 PM

aklim

Team Owner

Joined: Jul 2004

Posts: 28,475

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From: Hartford WI

Did you clean the IAC housing? I would take it all apart and clean it out throughly. Also, what were the IAC counts you get with a scanner? You should get between 20-30 counts and then you set the TPS to 0.54V. To check the TPS, you need to have the scantool setup to measure TPS voltage. See if there is a dropoff somewhere between 0.54V to WOT. If there is a dropoff, replace it. Also make sure the IAC wires are not loose. See what the meter tells you for IAC Counts after you cleaned the IAC housing.

Aug 1, 2004 | 08:03 PM

autotech

Thread Starter

Racer

Joined: Oct 2003

Posts: 273

Likes: 0

From: Midland Texas

The cam is a comp. 08-501-8

lift -
int - .488
ex - .495
duration @ .050
int - 212
ex - 218

It is not a lope though. I am going to sit down tommorrow ang go thru the IAC numbers and check wiring, min. idle, ect...

Aug 1, 2004 | 08:05 PM

autotech

Thread Starter

Racer

Joined: Oct 2003

Posts: 273

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From: Midland Texas

and check TPS numbers as well - I belive - IAC or TPS, one of the two.

Aug 1, 2004 | 08:24 PM

CFI-EFI

Race Director

Joined: Sep 2000

Posts: 17,298

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From: The Top of Utah

Quote:

Originally Posted by aklim

To check the TPS, you need to have the scantool setup to measure TPS voltage.

Darn! And to think I've been using a DVOM all this time.

autotech,
I agree, that's not a LOT of cam, but it could be contributing to the problem. I had assumed, that a tech would have the IAC squared away. With clean IAC passages, if it does lope, sometimes a greater than spec minimum idle will help. In extreme cases, I have heard of raising the minimum idle and not using the IAC, at all. Yours shouldn't be that bad. If all else fails, try a little advance. Heck, try some advance, anyhow.

RACE ON!!!

Aug 1, 2004 | 08:35 PM

autotech

Thread Starter

Racer

Joined: Oct 2003

Posts: 273

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From: Midland Texas

I have checked everything with the IAC, including cleaning. It just has been a while. I am going to re-check it. I dont want to put any timing in the motor because of detonation...If I dont run 92 octane this motor rattles hard...

Aug 4, 2004 | 01:43 PM

mvmechanic

Advanced

Joined: Feb 2004

Posts: 51

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From: Mission Viejo CA

I noticed that you said the motor is "new". How new is it? Has it always had the same problems? I would start at the very beginning: How's the compression and are there no vacuum leaks? Perhaps a leak down test is in order. What do the plugs look like? You mentioned that it pings; as well. Pinging and rough idle may both be caused by an intake manifold/vacuum leak. Hope this is helpful; good luck.