Minimal Vacuum at idle

DuckMan · 11-16-2001, 01:01 AM

I know this topic has been beat to death, but I'm at my wits end. I've checked for vacuum leaks, disconnected and plugged the supply line from the tree to isolate the engine, replaced the hoses on the underside of the intake (GT-40) and still have only 2 inches at idle. Of course the idle quality is terrible and the car is getting terrible mileage. what else can it be?
,
The car is an 88 GT with a 351W and an S-trim. F303 cam, MAC shorties, Bassani X-pipe, Pro-M 75, 36lb injectors, GT-40X heads, and was built as a blower motor from the start. I just had a mishap on the Duck so I really need the Stang to run. I've pulled the codes and get an 11 on the KOEO, and a 66 on the KOER. I know this indicates the MAF is not working correctly, but Pro-M thinks I should try and find the leak first before sending it back.

Any help is greatly appreciated, I am new to the forum, and look to be active. TIA

joe

DuckMan · 11-16-2001, 01:03 AM

Oops, one other thing, I also got a 98 code when running the KOER, I can't find a good definition for this one. Anyone? Thanks again.

joe

PKRWUD · 11-16-2001, 06:21 AM

Wow. Code 98. I don't see that one that often. It means there is a major failure with one of your sensors. When this occurs, the ECM disregards the info coming from that sensor and substitutes a pre-programed signal until the sensor is repaired or replaced. The way to determine which sensor has failed is to run both KOEO and KOER self tests, and the bad sensor will be identified. In your case, it's the MAF. Your MAF has failed, and the ECM is substituting a predetermined value in it's place (aka: "limp home mode"). Problem is, unless the ECM has a chip that was custom burned for your mods, it's defaulting to a value based on a stock engine. This means the ECM thinks you have 19# injectors, a stock cam, no blower, etc. The results would be a no-start or very poorly running condition. That being said, 2 inches of vacuum still seems way too low, even under those circumstances. The only thing I can think of is maybe your blower is creating boost at idle. I would check it and verify that there is no boost at idle, and then replace the MAF. Once the MAF is working properly, the code 98 will go away, and the ECM will accept real signals as opposed to preset ones. Once the code 98 is gone, your driveability should be restored.

Take care,
-Chris

DuckMan · 11-16-2001, 09:44 AM

Chris,

Thanks for the info, I guess the MAF goes back to Pro-Flow. In your reply, you mention verifying the blower is not making boost at idle, how would one go about doing that?

I did drive the car today and found at 65MPH, which translates to about 2200 RPM, and the throttle completely closed, eg. foot off the pedal and coasting, it will pull only about 10-12 inches, while down shifing, eg off-ramp, I'll see about 20 inches. Cruising, steady state, we're back to the idle numbers of 0 - 2 inches. I don't know if this provides any additional information, but I thought I'd list what I saw this morning. At least she idles, poorly, but idles now. Thanks again, I appreciate the input.

joe

11-16-2001, 01:01 AM	#1
DuckMan Registered Member Join Date: Nov 2001 Posts: 6	Minimal Vacuum at idle I know this topic has been beat to death, but I'm at my wits end. I've checked for vacuum leaks, disconnected and plugged the supply line from the tree to isolate the engine, replaced the hoses on the underside of the intake (GT-40) and still have only 2 inches at idle. Of course the idle quality is terrible and the car is getting terrible mileage. what else can it be? , The car is an 88 GT with a 351W and an S-trim. F303 cam, MAC shorties, Bassani X-pipe, Pro-M 75, 36lb injectors, GT-40X heads, and was built as a blower motor from the start. I just had a mishap on the Duck so I really need the Stang to run. I've pulled the codes and get an 11 on the KOEO, and a 66 on the KOER. I know this indicates the MAF is not working correctly, but Pro-M thinks I should try and find the leak first before sending it back. Any help is greatly appreciated, I am new to the forum, and look to be active. TIA joe

11-16-2001, 01:03 AM	#2
DuckMan Registered Member Join Date: Nov 2001 Posts: 6	Minimal Vacuum at Idle Oops, one other thing, I also got a 98 code when running the KOER, I can't find a good definition for this one. Anyone? Thanks again. joe

11-16-2001, 06:21 AM	#3
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Wow. Code 98. I don't see that one that often. It means there is a major failure with one of your sensors. When this occurs, the ECM disregards the info coming from that sensor and substitutes a pre-programed signal until the sensor is repaired or replaced. The way to determine which sensor has failed is to run both KOEO and KOER self tests, and the bad sensor will be identified. In your case, it's the MAF. Your MAF has failed, and the ECM is substituting a predetermined value in it's place (aka: "limp home mode"). Problem is, unless the ECM has a chip that was custom burned for your mods, it's defaulting to a value based on a stock engine. This means the ECM thinks you have 19# injectors, a stock cam, no blower, etc. The results would be a no-start or very poorly running condition. That being said, 2 inches of vacuum still seems way too low, even under those circumstances. The only thing I can think of is maybe your blower is creating boost at idle. I would check it and verify that there is no boost at idle, and then replace the MAF. Once the MAF is working properly, the code 98 will go away, and the ECM will accept real signals as opposed to preset ones. Once the code 98 is gone, your driveability should be restored. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

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11-16-2001, 09:44 AM	#4
DuckMan Registered Member Join Date: Nov 2001 Posts: 6	Chris, Thanks for the info, I guess the MAF goes back to Pro-Flow. In your reply, you mention verifying the blower is not making boost at idle, how would one go about doing that? I did drive the car today and found at 65MPH, which translates to about 2200 RPM, and the throttle completely closed, eg. foot off the pedal and coasting, it will pull only about 10-12 inches, while down shifing, eg off-ramp, I'll see about 20 inches. Cruising, steady state, we're back to the idle numbers of 0 - 2 inches. I don't know if this provides any additional information, but I thought I'd list what I saw this morning. At least she idles, poorly, but idles now. Thanks again, I appreciate the input. joe