94 sluggish when cold
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Instructor
Joined: Apr 2011
Posts: 209
Likes: 31
From: east of dallas
94 sluggish when cold
Got a 94 base coupe with 108k on it, car struggles for the first couple minutes when i get out the road, I even get a knock sensor code sometimes. Car runs perfect the rest of the day. Figure someone else has had a similar issue, any advice will be greatly appreciated.
Drifting
Joined: Apr 2020
Posts: 1,925
Likes: 610
From: Orange, CA
"I even get a knock sensor code"
This means your knock sensors are on the way out. Or rather, they're probably working fine--but the resistive elements inside them that tell the PCM they exist are failing, which is a super common issue. You can either replace the sensors (inexpensive, easy to do, a tiny bit messy since coolant will spew out once you remove the old ones), or using a cable and some free software just disable the knock sensor error code and run without that detection. I chose to go with the latter on my own car since I could see the knock sensors working even when the PCM was claiming they weren't there (again, very common issue on the pre-96 cars).
Since you have a 94, my recommendation is as always to grab one of those cables, plug a laptop into the car, and log a drive from dead cold to warmed up and then post the log here for analysis. That way we can see exactly what your engine computer is seeing, which should make it clear what exactly the problem is. If you would like to go this route, I'm more than willing to help analyze the log with you. You can get the tools you need here:
Cable: http://aldlcable.com/products/aldlobd2u.asp
Datalog Software: http://fbodytech.com/eehack-2/download-eehack/
(Optional) Read/Flash Tune: http://fbodytech.com/flashhack/#Downloads
(Optional) Make Tune Changes: https://www.tunerpro.net/downloadApp.htm
(Optional) Definition for TunerPro for 94/95 Corvette: http://fbodytech.com/eex-tunerpro-definition/
If you do not wish to go that route, that's fine too, there's always analog ways of diagnosing problems like these in addition to the digital ones. For example, if it really is determined by temperature and temperature alone, and the car just doesn't seem to run right until it's hot, the first port of call is your coolant temperature sensor. This is located on the front of the engine, hanging off the water pump. If it's sending incorrect temperature data to the computer, it will cause the computer to use the wrong fueling calculations, which can cause the engine to run improperly until the actual temperature starts to match the reported temperature. Though, to be honest, most of the coolant temperature sensors I've seen fail do so 'cold', not 'hot'. That is, when they fail, they constantly report colder-than-actual temperatures, not hotter-than-actual.
If the coolant temperature sensor is fine, then we move on to what's different between a cold engine and a warm one. The first big thing is that a cold engine runs in open loop (using onboard fixed calculations for fuel), and a hot one runs in closed loop (which corrects for changes in fuel requirements on the fly using O2 sensors). With that in mind, the problem would seem to be something that the computer can correct for when in closed loop, but cannot compensate for in open loop. Something like a downed injector, or slightly improper fuel pressure, or a vacuum leak could fall into this category. Something not catastrophically wrong, but wrong enough that it can't be accounted for until you're in closed-loop operation.
For me personally, I would want to datalog. This would tell you instantly if the coolant temperature sensor (and all the other sensors) are working correctly, and would also tell you what closed-loop fueling settings are being used to see whether the computer is adding or removing fuel and exactly how much and when.
Best of luck!
This means your knock sensors are on the way out. Or rather, they're probably working fine--but the resistive elements inside them that tell the PCM they exist are failing, which is a super common issue. You can either replace the sensors (inexpensive, easy to do, a tiny bit messy since coolant will spew out once you remove the old ones), or using a cable and some free software just disable the knock sensor error code and run without that detection. I chose to go with the latter on my own car since I could see the knock sensors working even when the PCM was claiming they weren't there (again, very common issue on the pre-96 cars).
Since you have a 94, my recommendation is as always to grab one of those cables, plug a laptop into the car, and log a drive from dead cold to warmed up and then post the log here for analysis. That way we can see exactly what your engine computer is seeing, which should make it clear what exactly the problem is. If you would like to go this route, I'm more than willing to help analyze the log with you. You can get the tools you need here:
Cable: http://aldlcable.com/products/aldlobd2u.asp
Datalog Software: http://fbodytech.com/eehack-2/download-eehack/
(Optional) Read/Flash Tune: http://fbodytech.com/flashhack/#Downloads
(Optional) Make Tune Changes: https://www.tunerpro.net/downloadApp.htm
(Optional) Definition for TunerPro for 94/95 Corvette: http://fbodytech.com/eex-tunerpro-definition/
If you do not wish to go that route, that's fine too, there's always analog ways of diagnosing problems like these in addition to the digital ones. For example, if it really is determined by temperature and temperature alone, and the car just doesn't seem to run right until it's hot, the first port of call is your coolant temperature sensor. This is located on the front of the engine, hanging off the water pump. If it's sending incorrect temperature data to the computer, it will cause the computer to use the wrong fueling calculations, which can cause the engine to run improperly until the actual temperature starts to match the reported temperature. Though, to be honest, most of the coolant temperature sensors I've seen fail do so 'cold', not 'hot'. That is, when they fail, they constantly report colder-than-actual temperatures, not hotter-than-actual.
If the coolant temperature sensor is fine, then we move on to what's different between a cold engine and a warm one. The first big thing is that a cold engine runs in open loop (using onboard fixed calculations for fuel), and a hot one runs in closed loop (which corrects for changes in fuel requirements on the fly using O2 sensors). With that in mind, the problem would seem to be something that the computer can correct for when in closed loop, but cannot compensate for in open loop. Something like a downed injector, or slightly improper fuel pressure, or a vacuum leak could fall into this category. Something not catastrophically wrong, but wrong enough that it can't be accounted for until you're in closed-loop operation.
For me personally, I would want to datalog. This would tell you instantly if the coolant temperature sensor (and all the other sensors) are working correctly, and would also tell you what closed-loop fueling settings are being used to see whether the computer is adding or removing fuel and exactly how much and when.
Best of luck!
Drifting
Joined: Apr 2020
Posts: 1,925
Likes: 610
From: Orange, CA
No they are not. The knock sensor code is only related to the resistive element inside the knock sensors that tells the PCM whether the sensor is physically present or not. When the voltage is outside of the specified range, the code trips.
It has nothing to do with knock, and nothing to do with how well the engine is running.
It means the elements have failed or are failing. Which as I mentioned, is very common.
This DTC is unrelated to your issue.
It has nothing to do with knock, and nothing to do with how well the engine is running.
It means the elements have failed or are failing. Which as I mentioned, is very common.
This DTC is unrelated to your issue.
Drifting
Joined: Apr 2014
Posts: 1,396
Likes: 129
From: Plainfield IN
replace the temperature sensor but also make sure the plug makes proper contact too - been there done that.
