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What the hell MAF is that? Definitely not OE. K&N instructions have you place the MAF where you have it so the location of it shouldn't be an issue unless you've over-oiled the filter element.
If your PCM isn't calibrated for that MAF you can certainly set a chain of events that could cause your issue. The first clue is the high knock counts. Your PCM's calibration hasn't been altered, since you're bone stock except the CAI, and you haven't mentioned a tune (change in the calibration file in your PCM). Incorrect air mass assumed by the PCM would cause incorrect fuel delivery and spark timing on tip-in or large throttle position change... wrong air, wrong fuel, wrong increase in air mass and there you are. And if your TPS voltage is significantly off it'll multiply the problem...
Did you cut the wires for the OE MAF to install that one or is there an adapter for it?
EDIT: Sorry, saw you said it's a brand new Delco MAF with 'screen' in it. Can you provide the part #?
If your K&N connections are not solid post MAF....you get un-metered air and the car will not run right. I put on the Impala K&N 24 years ago and it had a habit of the rubber coupler coming off MAF, especially when you floor it as engine torque pulled it away
I eventually went with the larger F body MAF and intake elbow but it was tuned for the larger MAF when I went H/C motor. Still have the same set up on my 383. I have since resolved the crap coupler connection that came with the K&N and also moved the MAF closer to TB for better throttle response. innovativewiring.com makes the MAF harness extension for this. The SS clamp/spacer is $3 at Home Depot
Backfires are typically caused by either spark happening at the wrong time or valves not fully closed/sealed when they should be.
First off, getting this car from an older guy means it was likely driven very easy. You might have carbon buildup on the valves, preventing a good seal between the valves and heads. I would do both a compression test and leak down test on each cylinder. Put a can of Seafoam in the gas tank and another partial can into the intake via a vacuum port, then go out and drive it like you stole it to blow a bunch of the carbon out.
Next onto the ignition, you have replaced the routine maintenance items, but there are other things too. The Opti's are known for getting moisture inside, this can cause corrosion on the wheel inside that spins past the optical sensor, which acts like a camshaft position sensor. Have you replaced the ignition module or ignition coil? The car may only have 77k miles, but some parts go bad from age. These are just some things I've seen cause backfires and ignition issues over the years. Good luck!
What the hell MAF is that? Definitely not OE. K&N instructions have you place the MAF where you have it so the location of it shouldn't be an issue unless you've over-oiled the filter element.
If your PCM isn't calibrated for that MAF you can certainly set a chain of events that could cause your issue. The first clue is the high knock counts. Your PCM's calibration hasn't been altered, since you're bone stock except the CAI, and you haven't mentioned a tune (change in the calibration file in your PCM). Incorrect air mass assumed by the PCM would cause incorrect fuel delivery and spark timing on tip-in or large throttle position change... wrong air, wrong fuel, wrong increase in air mass and there you are. And if your TPS voltage is significantly off it'll multiply the problem...
Did you cut the wires for the OE MAF to install that one or is there an adapter for it?
EDIT: Sorry, saw you said it's a brand new Delco MAF with 'screen' in it. Can you provide the part #?
The original MAF are metal but new OEM GM are now plastic LS style and yes they use adapter plug to work .
Regarding oiling of air filter I am pretty careful at doing this. I degrease first let dry on vent for a few days then lightly oil.
No I was going researching to maybe get another one since they are only like 60 bucks for AC Delco new. Do you think I can remove my original one and reuse gasket afterwards just to be able to test it?
If your K&N connections are not solid post MAF....you get un-metered air and the car will not run right. I put on the Impala K&N 24 years ago and it had a habit of the rubber coupler coming off MAF, especially when you floor it as engine torque pulled it away
it is a trickt set up I was super careful when i re installed everything.
Funny thing comes to mind years ago my old VW that had MAF not installed right did the same type of intake back fire so I know for sure a post MAF leak can make a lot of issues
I eventually went with the larger F body MAF and intake elbow but it was tuned for the larger MAF when I went H/C motor. Still have the same set up on my 383. I have since resolved the crap coupler connection that came with the K&N and also moved the MAF closer to TB for better throttle response. innovativewiring.com makes the MAF harness extension for this. The SS clamp/spacer is $3 at Home Depot
I might rig up the MAF direct to TB just for jokes to see if fixes anything.
Did you have this issue before the K&N install?
The car came with the kit all I did was clean it etc
Originally Posted by Sparkee
Backfires are typically caused by either spark happening at the wrong time or valves not fully closed/sealed when they should be.
First off, getting this car from an older guy means it was likely driven very easy. You might have carbon buildup on the valves, preventing a good seal between the valves and heads. I would do both a compression test and leak down test on each cylinder. Put a can of Seafoam in the gas tank and another partial can into the intake via a vacuum port, then go out and drive it like you stole it to blow a bunch of the carbon out.
I ran a full can of Sea foam into intake roughly 2 weeks ago. I did this at 2000 rpm and once can was done I shut off and let it sit for a hour. Then started it up and test drive. Funny thing, this was the first car that produced NO smoke. I have used sea foam into intake of many cars in the past and always had smoke.
Next onto the ignition, you have replaced the routine maintenance items, but there are other things too. The Opti's are known for getting moisture inside, this can cause corrosion on the wheel inside that spins past the optical sensor, which acts like a camshaft position sensor. Have you replaced the ignition module or ignition coil? The car may only have 77k miles, but some parts go bad from age. These are just some things I've seen cause backfires and ignition issues over the years. Good luck!
I have removed opti and added new cap and rotor (new delco parts). Plus I connected scope to confirm things. Looks to be all good there.
IC and coil are original they are cheap to replace I might
Been running that same K&N cold air intake for well over 10 years on my 95 9C1 with no issues. Original stock MAF with screen at the filter/airbox. If you have your original in good working shape, it's a simple swap as a test.
Jim
Last edited by JimCT_9C1; 08-15-2021 at 08:01 AM.
Reason: typo
Here's my concern about your replacement MAF. To illustrate, below is a comparison of a MAF from 99-02 trucks and the LT1 F-Body MAF. They operate on very similar frequency ranges but Hz to g/sec airflow is not the same. Without knowing the frequency Hz to g/sec airflow of your replacement MAF, I'd be very concerned about the Hz seen by your PCM not matching the actual g/sec airflow in the calibration. Neither of these examples below matches your factory MAF, unless the Impala SS came with the larger MAF like the F-Body. I'm pretty sure the Caprice, Roadmaster and Fleetwood came with the smaller MAF. If you have your original MAF and it's not damaged, clean it with MAF cleaner and try it and see if your issue goes away.
When I was chasing this I used some 4" PVC to just build a test tube for before and after the sensor.
Used flexible plumbing rubber couplers from home depot.
good to know thanks I will have a look tomorrow at MAF and hopefully MAP
GM made 2 different diameter MAFs for the LT1 alone. The replacement from GM is correct as long as the diameter is correct. The B/D cars and the 4.3 Vortecs used a 3" MAF. The F/Y car LT1 and the older SBC based Vortec V8s from 96-02 used the 3.5" MAF. Using the larger MAF in place of the smaller one without changing the tune will make the car run incredibly lean.
In re-reading this thread I noticed you changed injectors. Did you run/test the originals after the fuel pressure correction and was there an issue? Asking since the new injectors could have different operating characteristics than the oem stockers. This can lead to similar issues as discussed in earlier posts for MAF differences.
If the originals are in good shape, they can be swapped as a test. I'm still running the original injectors on my 95 9C1.
If the MAF table is unreasonable, I think it will show up in the long term fuel trims. Closed loop should catch and try to correct that fueling error. His LT trims are very reasonable, so I doubt the MAF calibration is his backfire problem. Same with the injector sizing. However that does raise one question......do the Bosch injectors have the correct spray pattern? Need to make sure they are spraying for a single intake valve application and not a dual intake valve application(split pattern).
Don't like that cheesy looking plug they used on the 90 elbow off the throttle body. Check that for leaks. That could be letting in unmetered air.
That plug is basically like a hockey puck and its very thick so the fit is very good I was able to test it some more today with brake cleaner etc. it is sealed for sure.
Originally Posted by Fast355
GM made 2 different diameter MAFs for the LT1 alone. The replacement from GM is correct as long as the diameter is correct. The B/D cars and the 4.3 Vortecs used a 3" MAF. The F/Y car LT1 and the older SBC based Vortec V8s from 96-02 used the 3.5" MAF. Using the larger MAF in place of the smaller one without changing the tune will make the car run incredibly lean.
The Diameter is dead on like stock no issues to fit.
Originally Posted by JimCT_9C1
OP -
In re-reading this thread I noticed you changed injectors. Did you run/test the originals after the fuel pressure correction and was there an issue? Asking since the new injectors could have different operating characteristics than the oem stockers. This can lead to similar issues as discussed in earlier posts for MAF differences.
Originally when I was working on fuel system correction I did Fuel pump, Filter and FPR.
I changed the injectors afterwards simply because I read it was common for LT1 to have injectors fail so I thought why not do it now.
If the originals are in good shape, they can be swapped as a test. I'm still running the original injectors on my 95 9C1.
Just another thought -
Jim
Originally Posted by ACE1252
If the MAF table is unreasonable, I think it will show up in the long term fuel trims. Closed loop should catch and try to correct that fueling error. His LT trims are very reasonable, so I doubt the MAF calibration is his backfire problem. Same with the injector sizing. However that does raise one question......do the Bosch injectors have the correct spray pattern? Need to make sure they are spraying for a single intake valve application and not a dual intake valve application(split pattern).
So a little update . I didn't have much time today but I did a few basic testing .
I connected stock MAF and its the same regarding intake back fire. (See vid) It was a little rough at first but that could be from the MAF cleaner. I do feel it ran better with the new MAF for sure.
I sprayed brake cleaner all around intake piping EGR etc no change in rpm.
Now onto basic checking of the EGR. I was able to test diaphragm by pushing on it and it would change rpm so its not stuck. I tried this at idle and it would stall car out but if I raised rpm it was easier to keep car running. so thats good.
I was going to test the vacuum port next. I got vacuum line off and noticed the port was clogged. From what I see online the port hole is super tiny
but I still think my is jammed.
FWIW I did try one of those newer plastic MAF w/adapter, years ago. Car ran fine. Maf size and injectors are specific to the PCM tune. Even though a "same shape" one but different brand or lb rating will not have the same pulse width as stock and will cause trouble without a re-tune. Most tuners are familiar with the Ford SVO 30lb ones many of us run in H/C or 383 motors but the tune is specific for those injectors.
Original injectors can be serviced and flow tested. Typically $12-$15 each. I have used crizinperformance.com a few times with excellent results. Rich is a good guy and you get a full B/A report with his service
FWIW I did try one of those newer plastic MAF w/adapter, years ago. Car ran fine. Maf size and injectors are specific to the PCM tune. Even though a "same shape" one but different brand or lb rating will not have the same pulse width as stock and will cause trouble without a re-tune. Most tuners are familiar with the Ford SVO 30lb ones many of us run in H/C or 383 motors but the tune is specific for those injectors.
Original injectors can be serviced and flow tested. Typically $12-$15 each. I have used crizinperformance.com a few times with excellent results. Rich is a good guy and you get a full B/A report with his service
The MAF I have is more then just a match in size its what GM calls for so thats good enough for me.
Yes I was thinking to rebuild OEM but I just prefer new and Bosch is king with injectors. I built a few turbo cars in the past we always used Bosch.
Either way MAF and Injectors are not my problem simply because the back fire happened with OEM original parts and also with replacement parts.
Didn’t see you mention the outcome of the TPS signal as seen on the scope. Fast355 suggested it. Very important when you do it is to sweep through its range and look for jumps and/or dead spots. Also check to see if the adjustment is off. Though there is no ‘adjustment’ to a factory TPS you can slot the bolt holes slightly to get it to within 1/10V or so. shbox.com has a how-to on that if you want a visual/walkthrough of it.
Didn’t see you mention the outcome of the TPS signal as seen on the scope. Fast355 suggested it. Very important when you do it is to sweep through its range and look for jumps and/or dead spots. Also check to see if the adjustment is off. Though there is no ‘adjustment’ to a factory TPS you can slot the bolt holes slightly to get it to within 1/10V or so. shbox.com has a how-to on that if you want a visual/walkthrough of it.
Going to connect scope tomorrow I was able to open up diagram looks like I will be tapping into blue wire.
Many good points on here so far. Yes a MAF difference will cause major issue due to frequencies of the 2 sizes. Great point there.
As for spraying brake cleaner, listening for a change isn't good enough. You need to be looking at a scan tool watching for a change in O2 readings. Lean/rich. Even the smallest leak will cause issues, more-so when under a load.
Make sure you have a full 5V on the gray wire first, and double check the ground on the black wire. Then do a sweep test (slowly) on the blue wire. closed throttle should be .5V - .75V, sweep should go above 4V, ideally very close to 5V. Look for jumps/ dead spots.
Make sure you have a full 5V on the gray wire first, and double check the ground on the black wire. Then do a sweep test (slowly) on the blue wire. closed throttle should be .5V - .75V, sweep should go above 4V, ideally very close to 5V. Look for jumps/ dead spots.
Performed a little testing today.
First I like to comment on EGR. The port on it I thought was clogged but turns out the hole is less the 1/16". I was able to get a needle in there and it is working fine.
Holds vacuum and moves freely
Next up was TPS testing. I was able to see OL on meter in one location (see videos) and let me know your thoughts
-Ground (black) was tested first all good
-Positive side (grey) was giving me 5,08v so its fine too.
-Dark blue is signal wire. Closed TB was 0.73 to WOT 4.64v
08-14-2021, 08:48 AM
