Speed sensor cause this? - ZR-1 Net Registry Forums

LantanaTX · 04-25-2014

I did some searching and was wondering if a bad/weak speed sensor could cause, inconsistent idle down when at a stop, no cruise control and inconsistent opening of the secondaries?

When I come to a stop the engine most of the time remains at a high idle but usually finally drops while sitting at a stop light.

secondaries don't seem to always be opening. I have good vacuum.

cruise control doesn't do anything at all. fuse is good.

does the speed sensor throw a code?

What code reader do you recommend? any OBD-1 work? If I use the paperclip method. is the flashing in the LCD disaplay or the check engine light. I also need to figure out why my check engine light does not work. Could it have been disabled somehow? When I turn the key to the on position it does not light up on the information panel.

Thanks for the help guys!

tpr60 · 04-25-2014

High Idle in a 90 is common, GM did a updated prom to take care of this problem. I use this prom in one of my Zs and it works most of the time. Speed sensor if not working will set a code. Looks like someone removed the check eng. light bulb/or its burnt out. The light should come on when you turn the Ign. on as a bulb/circuit test, then turn off. You will not be able to do code test with a paper clip until you fix the bulb. I use a Snap-on scanner or most members use a Tech-1.

LantanaTX · 04-25-2014

Quote:

Originally Posted by tpr60

High Idle in a 90 is common, GM did a updated prom to take care of this problem. I use this prom in one of my Zs and it works most of the time. Speed sensor if not working will set a code. Looks like someone removed the check eng. light bulb/or its burnt out. The light should come on when you turn the Ign. on as a bulb/circuit test, then turn off. You will not be able to do code test with a paper clip until you fix the bulb. I use a Snap-on scanner or most members use a Tech-1.

I have a 1991 not '90. ECM says remanufactured by GM on it. Should I look at the prom and get a number off it?

tpr60 · 04-25-2014

First things first. Need to fix check engine light so you can pull codes with a paper clip, or borrow/buy a scanner. Either way the light fix is a starting point. Buying a factory service manual set is very important. Be careful removing the upper and lower trim around the radio/shifter to get to the check eng. bulb display.

LantanaTX · 04-25-2014

Quote:

Originally Posted by tpr60

First things first. Need to fix check engine light so you can pull codes with a paper clip, or borrow/buy a scanner. Either way the light fix is a starting point. Buying a factory service manual set is very important. Be careful removing the upper and lower trim around the radio/shifter to get to the check eng. bulb display.

Thanks, will check bulb. I do have the 1991 FSM as well as the ZR1 supplement.

tpr60 · 04-25-2014

Did the CEL ever work since you owned the car??

LantanaTX · 04-25-2014

Quote:

Originally Posted by tpr60

Did the CEL ever work since you owned the car??

No but only owned for two weeks. I did have to have it inspected which included the sniffer and it passed.

mike100 · 04-25-2014

The BFXB prom code is the latest for the 91 MY which included the idle and hot-start fixes. That said, the LT5 still cranks longer when warm and definitely has what some refer to as 'idle hang'. It idles up until you go below about 4 mph then finally drops to around 800 or so.

I have found that lowering the idle target speed at warmed up coolant temps helps this a little. Also may of these engines have a little bit of a throttle body leak or maybe another vacuum leak that kind of exasperates the ecm's ability to reach the desired idle speed. that can be checked by monitoring the IAC counts if you have a scanner or tech1. In my case, I custom made a PCV vent bypass to offset some amount of air going back to the plenum helping offset the throttle body wear leak.

mike100 · 04-25-2014

Nobody really can say why the idle scheme is so complex and why the idle hang exists, but one of my theories is that the heavy engine and wider front tires needed more power steering pressure.

LantanaTX · 04-25-2014

Quote:

Originally Posted by mike100

Nobody really can say why the idle scheme is so complex and why the idle hang exists, but one of my theories is that the heavy engine and wider front tires needed more power steering pressure.

Thanks for the input. Just for grins, I looked at the prom. Looks like FXFK. someone wrote "91 Production LT5" on the side of it where it plugs in with a sharpie.

Sometimes it is a few seconds after coming to a dead stop that the idle works it way down.

04-25-2014	#1
LantanaTX Join Date: Mar 2014 Location: Lantana, TX Posts: 134	Speed sensor cause this? I did some searching and was wondering if a bad/weak speed sensor could cause, inconsistent idle down when at a stop, no cruise control and inconsistent opening of the secondaries? When I come to a stop the engine most of the time remains at a high idle but usually finally drops while sitting at a stop light. secondaries don't seem to always be opening. I have good vacuum. cruise control doesn't do anything at all. fuse is good. does the speed sensor throw a code? What code reader do you recommend? any OBD-1 work? If I use the paperclip method. is the flashing in the LCD disaplay or the check engine light. I also need to figure out why my check engine light does not work. Could it have been disabled somehow? When I turn the key to the on position it does not light up on the information panel. Thanks for the help guys! __________________ [SIGPIC][/SIGPIC]1991 #0663

04-25-2014	#2
tpr60 Join Date: Feb 2011 Location: Salinas, Ca Posts: 161	Re: Speed sensor cause this? High Idle in a 90 is common, GM did a updated prom to take care of this problem. I use this prom in one of my Zs and it works most of the time. Speed sensor if not working will set a code. Looks like someone removed the check eng. light bulb/or its burnt out. The light should come on when you turn the Ign. on as a bulb/circuit test, then turn off. You will not be able to do code test with a paper clip until you fix the bulb. I use a Snap-on scanner or most members use a Tech-1.

04-25-2014	#4
tpr60 Join Date: Feb 2011 Location: Salinas, Ca Posts: 161	Re: Speed sensor cause this? First things first. Need to fix check engine light so you can pull codes with a paper clip, or borrow/buy a scanner. Either way the light fix is a starting point. Buying a factory service manual set is very important. Be careful removing the upper and lower trim around the radio/shifter to get to the check eng. bulb display.

04-25-2014	#6
tpr60 Join Date: Feb 2011 Location: Salinas, Ca Posts: 161	Re: Speed sensor cause this? Did the CEL ever work since you owned the car??

04-25-2014	#8
mike100 Join Date: Feb 2011 Location: San Marcos CA Posts: 1,802	Re: Speed sensor cause this? The BFXB prom code is the latest for the 91 MY which included the idle and hot-start fixes. That said, the LT5 still cranks longer when warm and definitely has what some refer to as 'idle hang'. It idles up until you go below about 4 mph then finally drops to around 800 or so. I have found that lowering the idle target speed at warmed up coolant temps helps this a little. Also may of these engines have a little bit of a throttle body leak or maybe another vacuum leak that kind of exasperates the ecm's ability to reach the desired idle speed. that can be checked by monitoring the IAC counts if you have a scanner or tech1. In my case, I custom made a PCV vent bypass to offset some amount of air going back to the plenum helping offset the throttle body wear leak.

04-25-2014	#9
mike100 Join Date: Feb 2011 Location: San Marcos CA Posts: 1,802	Re: Speed sensor cause this? Nobody really can say why the idle scheme is so complex and why the idle hang exists, but one of my theories is that the heavy engine and wider front tires needed more power steering pressure.