Popping / Backfires?

[ViciousVendetta]

Hey guys small question here... I just got a rebuilt 6 bolt slapped into my car, and shes running pretty well. Idle could be better, but that will be fixed in time. My one concern now is what I fear to be detonation / knock. Under acceleration the car feels fine, I havent opened her all the way up yet since I only have around 400 miles on the new motor. But under deceleration there is a noticeable amount of loud popping, or backfiring, sounds like its coming right from under the car, assuming most of the noise comes out the exhaust. At first is only would do that in 2nd / 3rd gear when letting the car slow itself down between 4k-2k rpm, but now it even does it up in 5th gear, and its starting to scare me. I have a pocket logger, and the trims seem pretty decent but my Low Trim is showing 139%, I tried changing my base on my GM MAFT which didnt work well so I will spend more time tuning. But the funny thing is that its reading 0 knock whatsoever, and isnt it fairly normal for any car to knock now and then? Im wondering if maybe theres a problem with my knock sensor for it to not be reading anything at all, or is that common? Of course im running premium gas, this car never used to have abnormal popping under deceleration until after the motor was rebuilt. Any ideas / fixes would help me alot, thaks guys!

 

[ViciousVendetta]

Also forgot to mention a funny thing with the idle... When I push the clutch in, the idle actually drops around 200 rpm, and it goes back up when the clutch is let out. It never used to do this, even after I got my new clutch and lightened flywheel. Its only done this ever since I got the new longblock... Any ideas? Also any insight into the strange backfiring under deceleration would help me alot, thanks

 

[TotalEclipse112]

RPM drop with the clutch in sound fine, its a hydraulic clutch with a vacum assist which will put a slight load on the motor and drop the RPM's. As for the sounds from under the car, are you still running a cat? If so at 139% you are running pig rich and not burning off all the gas you are dumping into the motor. If you have a cat it is doing its job by burning off all the excess gas/hydrocabarbons which are in your exhaust, that is probally the sound you are hearing is a combustion process taking place in the cat. This is assuming however that you still have a catalytic converter in place.

 

[ViciousVendetta]

Oh yea good catch, I actually do not have a cat. So is that bad then? ><

 

[TotalEclipse112]

It could be depeng on the excess amout of hydrocarbon vapors that have passed throught it. When excess fuel passes through it the platinum element inside it is supposed to force the fuel to burn off before it exits, reducung emissions, so its sounds like its doing what it is supposed to do. But if that is the case don't worry about detonation, your too rich for it. Try to lower your fuel trim and see what happens. If the noise goes away you should be alright, just make sure that your exhaust dosen't smell like rotten eggs or the chances are the cats had it.

 

[steve]

RPM drop with the clutch in sound fine, its a hydraulic clutch with a vacum assist which will put a slight load on the motor and drop the RPM's.

I've never seen a vacuum booster on a 1G clutch master. Where are you getting this information from?

The RPM drop could be from a malfunctioning ISC, IPS, or VSS (Vehicle Speed Sensor)

If so at 139% you are running pig rich and not burning off all the gas you are dumping into the motor.

With a low trim of 139% the ECU is pinned at it's max adding fuel because the O2 feedback is saying the car is running lean not rich. On a 1G numbers <100% mean the ECU is taking away fuel and number >100% mean the ECU is adding fuel.

Steve

 

[TotalEclipse112]

With a low trim of 139% the ECU is pinned at it's max adding fuel because the O2 feedback is saying the car is running lean not rich. On a 1G numbers <100% mean the ECU is taking away fuel and number >100% mean the ECU is adding fuel.

Whoops thinking of something else, wife needs to leave me alone and let me concentrate. Also if the car is adding max fuel wouldn't it create and excess in the exhaust stream? I know it sounds stupid but I had a car like this a couple of moths ago doing the same thing with a sound coming from under the car, from the cat. Turns out its LTFT and STFT were scewd up because of a bad o2 sensor and the car thought it was lean all the time so the cat did what he was describing, to eleminate the excess hydrocarbons in the exhaust stream.

 

[TotalEclipse112]

Hydrocarbons unite against CARB and the EPA

Sorry, bad attempt at humor.

 

[ViciousVendetta]

Again there is NO catalytic converter. And the plugs / wires are pretty new, engine was just rebuilt

 

[TotalEclipse112]

Whoops thought that you said you do have a cat. I'm sorry mis read on my part, maybe I need to repeate the 2 seconde grade or something, I'v been doing that a lot lately. Sound in your case could be and exhaust leak and thats the popping sound you are hearaing.

 

[TotalEclipse112]

I have a pocket logger, and the trims seem pretty decent but my Low Trim is showing 139%, I tried changing my base on my GM MAFT which didnt work well so I will spend more time tuning.

Thats what he said his trim was. And if it is an exhaust leak, that could trigger a lean condition. Either that ok like you said the o2 is toast.

 

[ddavisaf]

Hmmm.....
Bad ISC TPS or VSS....
Check them before you post back.
Running the codes will tell you if the VSS is faulty (on 2g's at least). No CEL will be present.

 

[ddavisaf]

My car stumbles/misses when I let off the throttle! Is there a fix?

The general consensus is that this is caused by the BOV, which can stick or plug. This appears to be especially true of the 2G BOV, although some 1G owners have had this problem as well.

Alexander Kowalski's Jan 27, 1999 fix to his off-throttle stumble went like this (edited for presentation):

"I took the BOV off a today for a closer look. I found some RTV plugging up a 3 mm diameter hole at the base of the BOV. This hole appears to be part of a passage in the BOV casting that travels straight up to the top. I am assuming it is some sort of return relief passage.

Not only did clearing the RTV solve my off throttle stumble, my BOV no longer sounds like a loud bird shriek between shifts. Its more like the soft 'phfft' sound I have associated with my two previous BOVs. Darn, other than scaring the heck out of my wife I really liked that sound."

Cleaning the BOV, replacing it or upgrading to a 1G unit should solve the problem. Owners of adjustable BOVs report that setting the BOV too tightly will cause this same problem, so a quick adjustment may be in order.

Owners who are having the problem with the engine RPM dropping abnormally low after letting off the throttle may be having problems with the speed sensor or idle switch on their cars. If one of these is malfunctioning the ECU may not realize the throttle is at idle until the engine RPM drops below the normal idle speed.

 

[ddavisaf]

And now, for my third post....
What kind of plugs and what are they gapped at?
I understand they're new.

 

[sciroccosven]

And now, for my third post....
What kind of plugs and what are they gapped at?
I understand they're new.

The popping sounds are common without a cat when you are relying on your engine to slow your car down. Your cat not only burns your gases clean but it creates positive pressure in your manifold and downpipe and quiets and controls the flow. Now that you have removed that you have nearly zero beckpressure and nothing to clean and burn out fuel and gases, and everything is now flying out as fast as it can. Im guessing its chaotic popping immediately after downshift and mellows out near 2,500 rpm. I wouldnt worry about that at all since your not under any load. The fact that your hearing that popping sound just means that your exhaust system is flowing as fast and freely as it possibly can and your probably making the most power you can from your exhaust. If you want peace of mind and some reassurance you should check your gap on your plugs and it wouldnt hurt to check them to see that they are all in good working order and show no signs of a problem. You also might want to consider replacing your o2 sensor. If you have any concerns relating to fuel that should be the first fix.

 

[ViciousVendetta]

You know the o2 sensor is very possible... According to my logger when i watch the car idle (which can go from 600-1200 at any given moment) the o2 trim varies wildly. It can go down to 60% when its idling low, and when the car is idling around 1000rpm the o2 trim is usually around 100%. Very erratic to say the least. Im not sure about the plugs, but I will buy wires and replace the plugs soon just to be careful, along with the o2 sensor. Thanks for all the replies, <3 you all!

 

[v8s_are_slow]

why has no one asked about his mid and low trims? what are those? what are your O2 readings? what about the timing? don't forget to throw in rpm's while you're at it so we can see what's happening at "x" time. you could be running rich. if you're at highway speeds, etc. you're not in the low trim anyway. so could pretty much throw the low trim out the window for this situation. you have a logger. give us more info. and is the logger throwing out any fault codes?

i also noticed you said you changed your base settings. what did you set everything too and what size injectors are you running? when you're changing your settings, how long are you waiting before you change them again?

 

[v8s_are_slow]

You know the o2 sensor is very possible... According to my logger when i watch the car idle (which can go from 600-1200 at any given moment) the o2 trim varies wildly. It can go down to 60% when its idling low, and when the car is idling around 1000rpm the o2 trim is usually around 100%. Very erratic to say the least. Im not sure about the plugs, but I will buy wires and replace the plugs soon just to be careful, along with the o2 sensor. Thanks for all the replies, <3 you all!

the o2 trim isn't the one you need to be looking at. of course it's gonna vary. the o2 trim is just to get a quick idea as to where your trims might be and then adjust accordingly. the main one's you need to be concerned with are the lows, mids, and high trims. what are those at?

watch your O2 voltage (not the trim, the regular O2's) and see if it cycles up and down while just cruising along and the pedal at a steady postition. once you go wot, it should stay still in a certain spot. if it's not cycling, yeah, the O2 might be bad.

and incase ya'll don't know already, plugs need to be gapped at .028. go with ngk's. the 6's which you should just be able to drive down to the dealer and buy them for cheap.

also, have the changed the thermostat? if so, what temp did ya put in? 190, 180, 160, etc.? do you have a coolant temp reading on your logger? if so, what is it? it should be a steady reading, not up and down.

 

[ViciousVendetta]

Looks like I need to do some good logging here, I was hopping this popping deal didnt have so many possible variables! So far the low trim is 139, mid and hi are both 103. But since it is still being broken in ive been driving her easy, havent even gone past 3.5k rpm. The only fault code is EGR, which has been getting thrown now and then since I got the car, the egr is of course blocked off, charcoal canister removed etc...

 

[v8s_are_slow]

Looks like I need to do some good logging here, I was hopping this popping deal didnt have so many possible variables! So far the low trim is 139, mid and hi are both 103. But since it is still being broken in ive been driving her easy, havent even gone past 3.5k rpm. The only fault code is EGR, which has been getting thrown now and then since I got the car, the egr is of course blocked off, charcoal canister removed etc...

you could stand to lean out the mid and high's some (around 110%-115% maybe depending on knock). that might help. but might be able to have a better idea if you're able to answer any of the above questions. for starters. these...

i also noticed you said you changed your base settings. what did you set everything too and what size injectors are you running? when you're changing your settings, how long are you waiting before you change them again?

 

[steve]

The only fault code is EGR, which has been getting thrown now and then since I got the car, the egr is of course blocked off.

Did you install a 1k resistor at R129? That will convert a 91-93 ECU from California to Federal.

Steve

 

[hellotbone]

My car did the backfire during idle every say 20 seconds it would pop and then nothing then pop again. I am running a cat and I'm 2G with 6bolt. Mine also in any gear when you let off pops no backfiring. Switched my 1G crushed BOV for a type RS no more probs. Not saying thats your problem but like others have posted have a look at it.