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Discussion Starter · #1 ·
so not only is this bugging the crap out of me but also the local mechanics. i want every gear head on here to put there thinking caps on good and tight and lets see if i can get some thoughts on this

96 impala, raiss intake, aftermarket maf.

car idles perfect runs smooth, drives great no problems at all exept when you turn the ac on. when you are just cruising down the road over 1500 rpm with the air on there is a feeling like a missfire every 4 seconds. even if you are at 2300 rpm the feeling is stronger but it is still every 4 seconds, ac off you cant feel it miss at all. if the air is on and you are accelerating you dotn feel the miss at all either

so we hooked the scanner up and there is no missfire on any of the 8 cyl. infact everythign on the scanner looked good exept the fuel learn ( i think thats what he called it) droped to no every time you felt the miss then went right back to yes. he is not thinking it is a miss though since the line was perfectly strait for all 8 cyl. its like somethign is telling the computer to turn off the fuel learn for that split second

so far i have put in new plugs, wires, sent computer out and had it reflashed ( so they say) did the egr check. there is no cel on so that dosent help much. he is beliving it is the computer but he had a spare one laying around out of a caprice i do belive, i pluged that one in and the cel came on rpms were 1000 off and it still had the same miss feeling.

any input would be appreciated b4 i go and buy a opti and that isnt even the problem
 

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Just to clairify, what kind of "aftermarket" MAF are you running?

If its not a 3.5" F-body MAF, thats a problem.

Tuning for the fancy, non-ACdelco MAF's most likely be moot, as they are designed as PCM "foolers", using the stock PCM MAF tables as a base.

They may seem OK on a bone stock car, but on a car with custom tuning and/or built engine, stick with the F-body MAF when you determine you need to upgrade.
 

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so not only is this bugging the crap out of me but also the local mechanics. i want every gear head on here to put there thinking caps on good and tight and lets see if i can get some thoughts on this

96 impala, raiss intake, aftermarket maf.

car idles perfect runs smooth, drives great no problems at all exept when you turn the ac on. when you are just cruising down the road over 1500 rpm with the air on there is a feeling like a missfire every 4 seconds. even if you are at 2300 rpm the feeling is stronger but it is still every 4 seconds, ac off you cant feel it miss at all. if the air is on and you are accelerating you dotn feel the miss at all either

so we hooked the scanner up and there is no missfire on any of the 8 cyl. infact everythign on the scanner looked good exept the fuel learn ( i think thats what he called it) droped to no every time you felt the miss then went right back to yes. he is not thinking it is a miss though since the line was perfectly strait for all 8 cyl. its like somethign is telling the computer to turn off the fuel learn for that split second

so far i have put in new plugs, wires, sent computer out and had it reflashed ( so they say) did the egr check. there is no cel on so that dosent help much. he is beliving it is the computer but he had a spare one laying around out of a caprice i do belive, i pluged that one in and the cel came on rpms were 1000 off and it still had the same miss feeling.

any input would be appreciated b4 i go and buy a opti and that isnt even the problem
its the ac compressor and or the pully.... when you have the ac on it cycles on and off. if you have the car idle you can hear the ac compressor turning on/off it makes aloud clicking sound.

that cycle is causing ur miss... it could be locking up, or fighting the belt (big problem)

or it could have a short and causing electircal issues.






you need the ecu tuend for a larger maf, or you will break your trans.


thanks
 

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Discussion Starter · #4 ·
the maf is a granitelli. wos on car when i bought it a month ago. as for the ac i dont think the compressor is bad, i think it is the load of the ac on that makes teh missfire feeling more prominent. one mech drove car and he said even with the air off he can still very very barely feel the miss
 

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Discussion Starter · #5 ·
also i disconected teh maf completely and drove the car around. it still has teh same miss feeling so i take it the maf has nothing to do with it.
 

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the maf is a granitelli. wos on car when i bought it a month ago. as for the ac i dont think the compressor is bad, i think it is the load of the ac on that makes teh missfire feeling more prominent. one mech drove car and he said even with the air off he can still very very barely feel the miss
umm ok....
ac compressor on = miss
ac compressor off no miss

but try pulling an ac unit from a known working car.
 

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What about taking the belt off, turn the AC on, of course it shouldn't even try to kick on but something to check to make sure it isn't electrical persay, of course don't go far like that, but the car will run with out the belt, see if the miss is still there. Also different loads on the motor, hence higher kPa will put the PCM into a different cell for BLM's, when scanning the car, you need to find out if it is dropping a cell where it could be making it run uber rich causing a miss or a bog rather or it could be lean causing knock and the timing is getting retarded.

Also the IAC can play a roll as well, it should bump the airflow if the AC compressor kicks on so the motor doesn't "bog", if the IAC is sticking at a certain point it can cause this. Have them check the counts on the IAC motor when it happens and see what it is doing.

AS stated above, if the MAF tables are tuned for the MAF that you have it will cause all sorts of problems as the PCM is calibrated for a 3 inch MAF.

Just some thoughts, good luck.


Also just reread your post, turn the AC on MAX it should run in theory all the time, is the same thing felt every four seconds, or is the compressor cycling like it is getting low on freon, causing a load every four seconds, as that is usually the cycle time of the AC compressor when it is low on freon
 

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Discussion Starter · #8 ·
it is not the ac cylcling, the ac compresor has nothinig to do with the problem cause it still shows the drop to no when the air is off, you can only feel it better when the ac is on because of the load. as for the maf it is a 3 inch but if i can find someone local with a stock one i would gladly put it on the car and test it out. the readings showed all good on the maf when the scanner was hooked up. plus like i said with the maf disconected teh car still drove the same and had the same miss feeling. the miss feeling isnt reall bad, infact im sure some people who arnt car savoys like us wouldnt even feel it but its there and it bugs the crap out of me the car pulls great and feels like it was when brand new, its only when you are at a cruising speed over 1500 rpms that you can feel it and the weird thing is the fact that it misses every 4 seconds exactly.
 

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Discussion Starter · #9 ·
also i remember when it was being scaned there was no sighn of knock at all, in fact everything on the scanner was exactly what it should be. we had the scaner hooked up for almost a hour driving around and it stumped both mechanics.
 

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car idles perfect runs smooth, drives great no problems at all exept when you turn the ac on. when you are just cruising down the road over 1500 rpm with the air on there is a feeling like a missfire every 4 seconds. even if you are at 2300 rpm the feeling is stronger but it is still every 4 seconds, ac off you cant feel it miss at all. if the air is on and you are accelerating you dotn feel the miss at all either
i was mis lead by the above.

maybe the voltage regulator...turning the ac causes alot more current.

our cars have the VR built into the alternator. try another alt
 

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I am assuming that the mechnic is using a typical scanner? ie not a tech 1 or datamaster, if this is the case the information will be limited, when it does the miss or sag, have them log it, the scanner should be able to do that at least, check these things during, after and up to the hesitation:

-Map value does it vary, if so by how much?
-What BLM cell is it dropping to when this happens?
-What are the IAC counts?
-Is there any Spark Retard? this could indicate knock?

If they are able to run a scan freeze the data and print it out, post it up on here so we can look at it.

We can guess all we want but we need more data. Also what are your mechanics going off of as within range values?
 

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I am assuming that the mechnic is using a typical scanner? ie not a tech 1 or datamaster, if this is the case the information will be limited, when it does the miss or sag, have them log it, the scanner should be able to do that at least, check these things during, after and up to the hesitation:

-Map value does it vary, if so by how much?
-What BLM cell is it dropping to when this happens?
-What are the IAC counts?
-Is there any Spark Retard? this could indicate knock?

If they are able to run a scan freeze the data and print it out, post it up on here so we can look at it.

We can guess all we want but we need more data.
QFT .
 

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Discussion Starter · #13 ·
ill have to wait till monday to have him scan it again. i cant remember word for word what it was called but the only thing that droped ( or went to no) was the fuel learn ..... something. lol i know that isnt helpfull but that was the only thing that had affect when the car made the miss. had no knock problems and all other sensors were reading correct. i did just get my hands on a stock maf and installed it with no change what so ever. i notice a split second b4 the compressor kicks on more air is gettign sucked into the intake and right b4 it kicks off i can hear the air not being sucked in as much. what sensor on the throtle body makes more air come in to compensate? im thinking of buying some new gaskets tomorow and cleaning out the throtlebody see what that does.

also is there any other way to put a load on the engine without having the ac on so i can rule the compressor being defective out? because like someone stated earlier the miss feeling happens as the compressor kicks on and off. and when you guys drive and turn youre ac on can you feel the engine bog down just a hair? and does ti do that every time the compressor cycles? my other cars never did it as the compresor cycles.
 

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ill have to wait till monday to have him scan it again. i cant remember word for word what it was called but the only thing that droped ( or went to no) was the fuel learn ..... something. lol i know that isnt helpfull but that was the only thing that had affect when the car made the miss. had no knock problems and all other sensors were reading correct. i did just get my hands on a stock maf and installed it with no change what so ever. i notice a split second b4 the compressor kicks on more air is gettign sucked into the intake and right b4 it kicks off i can hear the air not being sucked in as much. what sensor on the throtle body makes more air come in to compensate? im thinking of buying some new gaskets tomorow and cleaning out the throtlebody see what that does.

also is there any other way to put a load on the engine without having the ac on so i can rule the compressor being defective out? because like someone stated earlier the miss feeling happens as the compressor kicks on and off. and when you guys drive and turn youre ac on can you feel the engine bog down just a hair? and does ti do that every time the compressor cycles? my other cars never did it as the compresor cycles.
The fuel learn is the table where the BLM or Block Learn Modifier is located, it is a table in the PCM that is based on Airflow VS RPM for fuel compensation to keep the engine running at or close to stoich 14.7:1 AFR. So when there is a different load on the engine causing a higher kPa there will be more air drawn in it will go to another cell in this table, I think that I explained that right :) If there scan tool would be able to tell what cell it drops to as well as the BLM that is within that cell would be helpful.

The sucking more air in is an affect of the IAC (Idle Air Control) on the throttle body, it is a pintle that travels in and out to compensate for changes in the engine variables and to keep idle mainly, but it follows the throttle for unexpected or maybe expected loads, like AC and power steering, but these usually have an affect at idle or there abouts.

As far as "bog" when AC kicks on, I can feel an ever so slight bog in every Bbody I have been in. But maybe I am sensitive to this for some reason.
 
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