Chasing poor mileage issue
Chasing poor mileage issue Any ideas as to what could be causing my crap mileage?
Whats been done recently:
Entire ignition system >15k (Plugs/wires/delco opti/ ICM/ coil)
Injectors >5k, good fuel pressure.
TB/ MAF cleaned >5k.
O2s showing good numbers (128)
I thought I had an exhaust leak because I could see a tiny bit of smoke from under the car every once in a while, turns out it was a small oil drip on the y-pipe. (Fire hazard
)
Only things I can think of left to do is clean/ scan MAP numbers/ air filter (K&N CAI)/ fuel filter.
Does anyone know what the MAP should be reading? I can't find any numbers. The engine has been running great for a while but suffers from ghost knock (LT4 knock module) that I've been trying to track down (intake/ exhaust/ pcmforless tune- 10* knock at WOT regardless of that or stock tune), also gets an SES when cruising in 6th at 1500rpm or less, these are the only other issues.
Used to get 18/28 last year, now get 14/22
Whats been done recently:
Entire ignition system >15k (Plugs/wires/delco opti/ ICM/ coil)
Injectors >5k, good fuel pressure.
TB/ MAF cleaned >5k.
O2s showing good numbers (128)
I thought I had an exhaust leak because I could see a tiny bit of smoke from under the car every once in a while, turns out it was a small oil drip on the y-pipe. (Fire hazard
)Only things I can think of left to do is clean/ scan MAP numbers/ air filter (K&N CAI)/ fuel filter.
Does anyone know what the MAP should be reading? I can't find any numbers. The engine has been running great for a while but suffers from ghost knock (LT4 knock module) that I've been trying to track down (intake/ exhaust/ pcmforless tune- 10* knock at WOT regardless of that or stock tune), also gets an SES when cruising in 6th at 1500rpm or less, these are the only other issues.
Used to get 18/28 last year, now get 14/22
If you have issues with CEL and ghost knock you need to send a full data log to PCM's for less and let them figure out what is going on in the tune. If the parts are solid, meaning no exhaust leaks, false leans, correct injector sizes, FP, MAP MAF and tire and gear size all good in tune and you have swapped in all these parts my guess would be tune just because I had a bad tune once that I did the same thing with parts. Changed the tune and problem solved. Never had a PCM's for less tune and alot of people like and use them it could be a bad PCM or maybe they got something wrong. Log it and send it to them. They should be able to help you.
Correct stock injectors that were low mileage take offs from a 94 Trans Am.
I thought it was the tune myself, so a few weeks ago I swapped PCMS with a stocker (too lazy to get CATS and swap bins lol) and the same thing was happening. Consistent 3-4* knock at part throttle and 8-10* at WOT.
I have posted datalogs on ltxtech and a few experienced guys took a look and claimed everything looked normal apart from the obvious knock.
I'm going to try the 3 things listed next when I have some spare time. Any other things I could potentially try? Someone on ltxtech mentioned that sometimes some crud in the cooling system can build up on the knock sensor and cause the false knock. Would the timing constantly being pulled be causing poor mileage as well?
Thanks guys!
I thought it was the tune myself, so a few weeks ago I swapped PCMS with a stocker (too lazy to get CATS and swap bins lol) and the same thing was happening. Consistent 3-4* knock at part throttle and 8-10* at WOT.
I have posted datalogs on ltxtech and a few experienced guys took a look and claimed everything looked normal apart from the obvious knock.
I'm going to try the 3 things listed next when I have some spare time. Any other things I could potentially try? Someone on ltxtech mentioned that sometimes some crud in the cooling system can build up on the knock sensor and cause the false knock. Would the timing constantly being pulled be causing poor mileage as well?
Thanks guys!
Your mileage is not horrible for having a 12 bolt and sticky wide tires. If the logs are fine Id almost have to say it is what it is although you still should not be getting CEL I had the same amount of knock and it never threw CEL. Have you tried going back to your old tire combo? Your turning big wide tires through a larger dif. it's gonna be one of the biggest fuel mileage killers.
Your mileage is not horrible for having a 12 bolt and sticky wide tires. If the logs are fine Id almost have to say it is what it is although you still should not be getting CEL I had the same amount of knock and it never threw CEL. Have you tried going back to your old tire combo? Your turning big wide tires through a larger dif. it's gonna be one of the biggest fuel mileage killers.
The only things I've done that could impact mileage a little bit was go from bald 315 Sumitomos that I bought the car with to the same size Nitto 555s which are notably more sticky but it'd be crazy to have such a huge mileage loss from that alone?
Edit to say that the mileage started going to hell when my MSD opti started crapping out, I swapped it with a Chevy Performance (Delco w/ Mitsu sensor) one and it never came back.
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Try some old school diagnosis, measure idle and cruise vacuum. Both should match. I'm reading like u have the stock cam which should have like 18" vacuum at idle and cruise.
Low vac means leaks. If vac good in OD then using too much fuel and look from there.
Hope this helps,
cardo
Low vac means leaks. If vac good in OD then using too much fuel and look from there.
Hope this helps,
cardo
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Verdad Gallardo Thanks cardo, will do next time I scan. I really wish I had more time to play with the car and figure this out! To me I feel like I'm down on power a decent amount since the timing is constantly being retarded when I'm on the throttle 
I would almost bet you Broke the sensor in the water pump... That sensor will read like -41* outside temp & fuel milage will go Down!!! Better check the sensor on the water pump... Its for the PCM & the one on the head is for the temp gauge... Ive seen quite a few that have that sensor broke & milage in the shithouse LoL
I'll double check that but wouldn't the car not go into closed loop like that?
I hit an all time low last night filling up, 11.6mpg
Granted it was 100% city and a good amount of lead footing, but thats still pretty terrible.
I hit an all time low last night filling up, 11.6mpg
Granted it was 100% city and a good amount of lead footing, but thats still pretty terrible.
I took the car over to my friend's place and we started poking around with his dad. It turns out I had a DTC 32 (somehow I missed it when scanning) and the vacuum lines were out of whack. We fixed that up and the car can finally cruise under 1600rpm in 6th without setting off the SES light.
I'm hoping the mileage improves (it should from this fix) at my next fill up, but I still think there is something else going on. I never messed around in that area and my mileage kind of dropped off on its own. All sensors are reading fine (his dad is a mechanic so he confirmed).
My voltage bounces around a bit and reads on the low side (12.5-12.8) so it looks like an alternator is in order soon enough.
I'm hoping the mileage improves (it should from this fix) at my next fill up, but I still think there is something else going on. I never messed around in that area and my mileage kind of dropped off on its own. All sensors are reading fine (his dad is a mechanic so he confirmed).
My voltage bounces around a bit and reads on the low side (12.5-12.8) so it looks like an alternator is in order soon enough.
I thought I'd update since my job has had me super busy since I started the thread.
I'm still getting **** mileage, avg about 17.5 and its really killing me on my 80 mile a day commutes. I used to get 23-25 avg on the same commute before this happened
Fuel filter/ PCV had no affect. I think its my EGR system that might be holding me back, any way to check that off the list?
Another thing I have is a quick bog if I punch it off the line without dumping the clutch, just for .5-1 sec. Last week I got on the highway at 2am and did a run up to 110 and it stopped pulling/ the RPM started faltering and catching back up
. Very strange because apart from the 1st gear bog I mentioned the car pulls hard up to 100.
Thanks guys.
I'm still getting **** mileage, avg about 17.5 and its really killing me on my 80 mile a day commutes. I used to get 23-25 avg on the same commute before this happened
Fuel filter/ PCV had no affect. I think its my EGR system that might be holding me back, any way to check that off the list?
Another thing I have is a quick bog if I punch it off the line without dumping the clutch, just for .5-1 sec. Last week I got on the highway at 2am and did a run up to 110 and it stopped pulling/ the RPM started faltering and catching back up
. Very strange because apart from the 1st gear bog I mentioned the car pulls hard up to 100.Thanks guys.