Sluggish engine, high mileage
Thread Starter
Instructor
Joined: Dec 2017
Posts: 100
Likes: 5
Sluggish engine, high mileage
Just got an FRC with ~140,000 miles and it's sluggish to rev up. Air cleaner is good, spark plugs appear relatively new, and I've cleaned the MAF. No DIC codes related to engine or powertrain. Any ideas on what it could be?
I've searched the forum and see some posts about clogged cats or fuel filter, anything else?
TIA
Edit: Forgot to mention that this is only when revving up fast. If I slowly go up, it'll stay at 5-6k rpms no problem, but if I punch it, its sluggish to go up
I've searched the forum and see some posts about clogged cats or fuel filter, anything else?
TIA
Edit: Forgot to mention that this is only when revving up fast. If I slowly go up, it'll stay at 5-6k rpms no problem, but if I punch it, its sluggish to go up
Last edited by Carolina99; Feb 13, 2019 at 08:02 PM.
Drifting
Joined: Jul 2015
Posts: 1,844
Likes: 375
From: San Jose, CA
Clean the air filter if it's washable, or put on a new one. Clean the throttle body. Also, using an OBD2 reader, what does your MAF read for airflow at idle?
I assume it's revving up slowly in neutral?
I assume it's revving up slowly in neutral?
Instructor
Joined: Dec 2012
Posts: 214
Likes: 26
Make sure who ever previously had the vehicle wasn't only running low grade octane. 
I would burn as much fuel as possible get close to E, get some fuel cleaners then fill the tank with the highest grade octane level you can and reset the PCM so the fuel tables are reset. Because running low grade octane will degrade engine performance.
I would burn as much fuel as possible get close to E, get some fuel cleaners then fill the tank with the highest grade octane level you can and reset the PCM so the fuel tables are reset. Because running low grade octane will degrade engine performance.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
I would check the items others had mentioned...just because the MAF is clean doesn't always mean it's OK without doing some testing with diagnostic equipment. You might be better off in the long run paying for diagnostic time without having to fire the "parts canon" and then doing the work yourself or just start replacing the cheap parts first !!
Drifting
Joined: Aug 2014
Posts: 1,627
Likes: 175
From: North pole
Just got an FRC with ~140,000 miles and it's sluggish to rev up. Air cleaner is good, spark plugs appear relatively new, and I've cleaned the MAF. No DIC codes related to engine or powertrain. Any ideas on what it could be?
I've searched the forum and see some posts about clogged cats or fuel filter, anything else?
TIA
Edit: Forgot to mention that this is only when revving up fast. If I slowly go up, it'll stay at 5-6k rpms no problem, but if I punch it, its sluggish to go up
I've searched the forum and see some posts about clogged cats or fuel filter, anything else?
TIA
Edit: Forgot to mention that this is only when revving up fast. If I slowly go up, it'll stay at 5-6k rpms no problem, but if I punch it, its sluggish to go up
Melting Slicks
Joined: Sep 2011
Posts: 2,258
Likes: 543
From: Appleton WI
Out of curiosity, what are you comparing the rev performance to? (Other V8s, 4 bangers, supercars). How does the car actually perform? If it pulls hard, it just may be the way the car is programmed for the TPS (Throttle Position Sensor). There is some lag between mashing the go pedal and a high spike in RPM, which if I understand correctly, can be adjusted by a tuner.
My recommendation is you find someone with an LS engine (preferably another Vette) and compare. Even though the Vette is a high performance car, the LS engine won't wind up as fast as a flat plane crank engine (many supercars) or a smaller high performance engine.
My recommendation is you find someone with an LS engine (preferably another Vette) and compare. Even though the Vette is a high performance car, the LS engine won't wind up as fast as a flat plane crank engine (many supercars) or a smaller high performance engine.
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Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,595
Likes: 4,055
From: Cape Coral, Florida
Out of curiosity, what are you comparing the rev performance to? (Other V8s, 4 bangers, supercars). How does the car actually perform? If it pulls hard, it just may be the way the car is programmed for the TPS (Throttle Position Sensor). There is some lag between mashing the go pedal and a high spike in RPM, which if I understand correctly, can be adjusted by a tuner.
My recommendation is you find someone with an LS engine (preferably another Vette) and compare. Even though the Vette is a high performance car, the LS engine won't wind up as fast as a flat plane crank engine (many supercars) or a smaller high performance engine.
My recommendation is you find someone with an LS engine (preferably another Vette) and compare. Even though the Vette is a high performance car, the LS engine won't wind up as fast as a flat plane crank engine (many supercars) or a smaller high performance engine.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
Great info posted !!...negative fuel trims at idle that increase ( go + ) like yours at higher rpm looks like dirty air filter or bad MAF sensor OR fuel delivery issue (fuel pump or filter or injectors)...your car is screaming for fuel at higher RPM's ( +14% - +16% LTFT's)...the MAF numbers look kind of low at 3700 RPM but good at idle ...if you can drive it at WOT (BEST way to check MAF sensor) and see what the numbers are...minimum of around 190 grams/sec are good !!...as far as fuel delivery if the air filter and MAF are good we can look at fuel pump or filter...you have a fuel pressure gauge ??...OH, I have a new Volumetric Efficiency program I just picked up for my laptop a while back...if you can possibly take a snap shot of WOT peak RPM, MAF (grams/sec), outside air temperature, current baro (inches/mercury) OR feet above sea level and humidity from the Weather Channel...I can plug in all those numbers and see if your car is "breathing" properly !!...this will rule our MAF/filter or plugged cats !!
EDIT: MAP at idle looks perfect !!
EDIT: MAP at idle looks perfect !!
Last edited by C5 Diag; Feb 14, 2019 at 05:03 PM.
Tech Contributor
Joined: Dec 1999
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Great info posted !!...negative fuel trims at idle that increase ( go + ) like yours at higher rpm looks like dirty air filter or bad MAF sensor OR fuel delivery issue (fuel pump or filter)...your car is screaming for fuel at higher RPM's ( +14-16% LTFT's)...the MAF numbers look kind of low at 3700 RPM but good at idle ...if you can drive it at WOT (BEST way to check MAF sensor) and see what the numbers are...minimum of around 190 grams/sec are good !!...as far as fuel delivery if the filter and MAF are good we can look at fuel pump...you have a fuel pressure gauge ??...OH, I have a new Volumetric Efficiency program I just picked up for my laptop a while back...if you can possibly take a snap shot of WOT peak RPM, MAF (grams/sec), outside air temperature, current baro (inches/mercury) OR feet above sea level and humidity from the Weather Channel...I can plug in all those numbers and see if your car is "breathing" properly !!...this will rule our MAF/filter or plugged cats !!
YOU, are one smart guy! I love your diagnostics! EXCELLENT work!!!!! 
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Joined: Sep 2013
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From: Cape Coral, Florida
Melting Slicks
Joined: Sep 2011
Posts: 2,258
Likes: 543
From: Appleton WI
Now that we know the OP is familiar with the C5, it makes sense to dig deeper. We know his SOTP Dyno is calibrated. lol
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,595
Likes: 4,055
From: Cape Coral, Florida
I agree a rev is a rev. The rate at which the revs climb can be impacted by rotating and reciprocating masses and frictional and pumping losses. The OP stated that if he mashed the go pedal it was slow to respond. I wanted to know what he was using as a baseline. There are complaints about the sensitivity of the TPS.
Now that we know the OP is familiar with the C5, it makes sense to dig deeper. We know his SOTP Dyno is calibrated. lol
Now that we know the OP is familiar with the C5, it makes sense to dig deeper. We know his SOTP Dyno is calibrated. lol
Thread Starter
Instructor
Joined: Dec 2017
Posts: 100
Likes: 5
This is great info, thanks for the help so far. I was able to collect some data this morning. Temperature was 48 degrees, 59 percent humidity, 29.7 barometric pressure. It's about 300-350 above sea level here. The refresh rate on the data was a bit slow, but here's what I gathered for a couple runs. This was driving, WOT. Highest RPM point that recorded.
MAF: 120
RPM: 5900
MAF: 148
RPM: 5300
LTFT went from about -4 at idle up to around 20ish at full throttle high RPMS. I have more data...let me know if other numbers would help.
MAF: 120
RPM: 5900
MAF: 148
RPM: 5300
LTFT went from about -4 at idle up to around 20ish at full throttle high RPMS. I have more data...let me know if other numbers would help.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,595
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From: Cape Coral, Florida
Thanks for getting those numbers !!... my laptop is at home and I'm at work right now so I won't be able to run those numbers till tonight...don't want to keep you in suspense...LOl !!!...here's a couple videos on V.E. !!
Last edited by C5 Diag; Feb 15, 2019 at 08:50 AM.
Team Owner
Joined: Oct 1999
Posts: 31,976
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From: Guinness Its whats for B'fast JAWGA
CI 3-4-5-9 Veteran
Great info posted !!...negative fuel trims at idle that increase ( go + ) like yours at higher rpm looks like dirty air filter or bad MAF sensor OR fuel delivery issue (fuel pump or filter or injectors)...your car is screaming for fuel at higher RPM's ( +14% - +16% LTFT's)...the MAF numbers look kind of low at 3700 RPM but good at idle ...if you can drive it at WOT (BEST way to check MAF sensor) and see what the numbers are...minimum of around 190 grams/sec are good !!...as far as fuel delivery if the air filter and MAF are good we can look at fuel pump or filter...you have a fuel pressure gauge ??...OH, I have a new Volumetric Efficiency program I just picked up for my laptop a while back...if you can possibly take a snap shot of WOT peak RPM, MAF (grams/sec), outside air temperature, current baro (inches/mercury) OR feet above sea level and humidity from the Weather Channel...I can plug in all those numbers and see if your car is "breathing" properly !!...this will rule our MAF/filter or plugged cats !!
EDIT: MAP at idle looks perfect !!
EDIT: MAP at idle looks perfect !!
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,595
Likes: 4,055
From: Cape Coral, Florida
OK, ran both MAF readings and RPM's and the verdict is...YOUR ENGINE HAS A BREATHING PROBLEM !!....either air not getting in or not getting out !!...coupled with the positive fuel trims I believe your MAF is bad !!...you said you had cleaned it...if your fuel trims went more negative under load (rich condition) I would look for clogged cats/exhaust...your VE was 34% and 47%....80 % is what you want to shoot for !!...if the VE was good fuel delivery would be the direction you would want to go !!..,also we should look at MAF wiring first !!
Last edited by C5 Diag; Feb 15, 2019 at 09:57 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,595
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From: Cape Coral, Florida
To check MAF wiring remove the connector..with KEY ON ENGINE OFF you should have 5 volts on the yellow wire, 12 volts (or battery voltage) on the pink, and less than 100mv (.1 volts)on the black/white wire !!
Last edited by C5 Diag; Feb 15, 2019 at 09:58 PM.