alright, so on my way home the other day I'm cruising about 75 when my speedometer dies. shortly there after i hear what sounds like some one pulling the pin on a grenade and throwing it under my car, only without all the shrapnel metal grinding mechanical failure kind of sounds. think depth charge blowing up under water. i get home (after babying the rest of the way) and find my coast clutches in my trans no longer seem to be working as the rpms drop back to idle and occasionally try to stall while braking and slowing down. performance doesn't appear to be affected, but it does sound like the cars exhaust got neutered. additionally it smells different then normal not a sulfur smell but definitely not normal and it's noticeable.

i check fluids to make sure nothing got mixed and all fluids look good (though tranny fluid smells slightly burnt, but is still bright red) code came back as 24 (vss not working) and 32 (EGR valve, EGR Solenoid, MAP). 24 is a no brainer, and i don't have reason to suspect 32 since my old z28 tripped it all the time and it was a false positive caused by the exhaust. i looked under it and didn't see any damage of fluids leaking. i believe fuel pressure to be fine as it fires up first time every time without any hesitation or issues but i haven't hooked it to a gauge yet to verify)

bottom line i'm at a loss. i was leaning toward bad catalytic converter except typically when those go you get a sulfur (rotten egg) smell and it gets louder... mine got quieter and no sulfur smell.

fuel economy has always sucked on this car also since i bought it. i get roughly 16-18MPH average before the gear change and about 18-20 after the gear change (which also doesn't make a lot of sense) where as a guy local with a 91 Vert gets around 22-24.....

Thanks

 

is it possible the VSS died and then the computer got confused, dumped extra fuel, raw fuel went into the exhaust and ignited? are any of the exhaust pipes fatter then normal or the muffler? im not sure how the computer collects the parameters for the amount of fuel to put in on the TPI but this seems possible to me

 

I would imagine your mpgs are suffering because of the bigger rims on your car. Or were you getting that mpg before putting the rims on?

Did you have the egr code before the noise?

Did it make the noise more than once?

 

Sounds like something in your trans let go. The coasting affects different internal clutches than acceleration I think. Dont be surprised if the trans starts behaving worse.

Im not an expert on auto transmissions though so I cant really say anything else. But I've heard of people saying that before and some responses were that something internal to the trans had failed.

Probably has something to do with your vss.

Mileage may suck because the trans is in sad shape (ie slipping)... possible.

 

i've wondered this. supposedly (according to the shop that does alot of work for me) the VSS shouldn't effect the fuel tables or any thing regarding how the engine runs and functions. i know this isn't completely accurate because when i first changed out to 3.73 gears from 2.73's i hit 109-ish on the speedometer (roughly 80 real speed because the VSS gear hadn't been changed) and the governor kicked in. if it has any further effect i'm not sure. which is why i'm seeking answers. either way i need to get the speedometer working again and have a post in the transmission forums regarding the drive and driven gears that as of yet hasn't been answered.

as for any pipes being fatter. nope they all look normal and nothing out of the blue.

I don't know the MPG on the car before the rims, as they were on the car before i bought it, but the overall diameter of the rim and tire is only 1" more than the stock 16" rim and tire. i couldn't imagine that would effect MPG as drastic as it appears, i could see tire width as there would be more contact surface, but my 20's are 245's which is the stock width. (could be wrong though).

Yes.... it's been tripping since i bought it in august. only seems to pop when i barely have my foot on the throttle to cruise which is consistent with my old Z28 (and was a false positive due to the exhaust being less restrictive).

nope so far just once, but it was loud enough and out of the norm i started babying the hell out of it and limping it to try to avoid any other damage.

the shop i've been doing business with for many years claims the coast clutches failing won't effect the rest of the trans or cause any damage. i have my doubts on that, but that's what they told me. seems to me if it fails it's only a matter of time before the rest of it does. much as i posted earlier i'm working on trying to fix the VSS and have a question posted in the transmission forum, but thus far have gotten no answer.

I've been wondering about this as it seems to slip WAY more then i would think it should, i've always had manuals and prefer them, but i pulled the trigger on the auto because getting a clean vert at a reasonable price is getting harder to do. so i figured i'd swap it to a T56 down the road.

 

That may well be true. I dont know much about automatics. It's still broken and I'd still consider that a problem... but it may not affect anything else at least. I'd be inclined to trust them on that. THe question is why did the coast side clutches suddenly stop grabbing? Could be part of a bigger issue... Which lead me to ...

My 305 TBI car got around 19mpg city driving. I dont think your mileage is THAT terrible. For a stock v8 TPI car I would expect a few more mpg's... but not a lot. Could be due to anything... Slipping trans is a definite possibility if it feels sluggish though.

 

when you stomp the gas it hesitates alittle like my Q-Jet IROC does when it's opening the secondaries, but once it gets what it needs it's takes off so it's not slipping in that sence, just when i'm casually driving or cruising i hit the pedal and it'll rev 500 rpm before it seems to grab and accellerate. i agree with the it's broken there's a problem and if it broke there has to be a reason so what is it. the main point of this post was to see what could have caused my explosion sound, but i do agree the trans issue needs to be resolved.

as for my mileage i guess i'm spoiled and see something wrong because my old 91 Z28 5 speed FRC car got 24MPG. i originally thought duh it's a vert, it's heavier and has a huge amount of drag when the top is down..... BUT there's a guy local in town with a 91 Z28 Vert and he claims to be getting what my 91 Z28 FRC used to. only really i suspect something is wrong with the mileage.

 

Nitroglycerine in the diff oil?

 

Maybe the ignition died briefly for whatever reason, dumped unburnt fuel into the pipe, came back to life, and detonated in the pipe. Don't know if this would happen with fuel injection, but I've had it happen with a carb. It blew the pipe apart at a joint.

I'm carbed, I only dream of getting that

 

Another possibility...

Do you have a posi? Could be something in the carrier that let go... Maybe the spider gears? Jack up the rear diff for a minute and see if the wheels still turn the same direction.

Whats so strange to me is cruising on the interstate at constant speed is one of the LEAST stressful things you can do to a car.

 

it can happen with EFI if the conditions are right.

Trust me i feel ya. my IROC is factory carbed with 3.73 gearing and i'm lucky to get that kind of mileage if i baby the throttle.... which is one reason i stopped using it as my DD and was grateful to have an EFI version as my DD.

i agree, i got a video of me spinning the wheels. i get a squealing that i don't think is right, but i just had the rear end rebuilt (again.... as the carrier went bad right after the new 2.73 gears were broken in) with brand new Yukon 3.73 gears, bearings seals, and a Yukon duragrip carrier..... there shouldn't be anything wrong with the rear-end..... but as you can hear in the video... this is with the e-brake off and the trans in neutral. it doesn't sound like it's coming from the brakes, and both tires do spin the same direction when spun. hard to spin in reverse then forward though.

http://s477.photobucket.com/albums/r...05_205359Z.mp4

 

That does sound kind of odd... It could be the brakes though. I'd jack it up and pull the calipers and see if it still does it. If so, I'd remove the driveshaft and repeat. Then I'd pull the differential cover and inspect it very closely. You may want to just do that anyway but we all know how stinky gear oil is...

 

To be clear, does the exhaust still sound exactly the same? Also when in park or in neutral does the engine rev normally when u mash the pedal? If its louder that would indicate the guts blown out of a cat or a hole in the exhaust system. Just asking because I've worked on cars where leaking injectors caused fuel to pool in the cat or even work its way back into the muffler and then go boom once the exhaust reached a hot enough temperature. In some one case the muffler actually had a hole blown in it. Other times there would be nothing noticeably wrong with the exhaust but when checking fuel pressure with the engine not running pressure would drop down fairly quickly due to a/some bad injector(s).

 

I'll have to wait till later this week/weekend to pull it all off. i have rear drums, and it sounds more like it comes from the center of the car then the outer edges, but i'll check anyway. I'm beginning to lean toward bad transmission, as my idle problem goes away when braking if i shift the trans into neutral to slow down and then drop it back in OD when i need to accelerate.

exhaust sounds the same at times but not always. sounds the same when starting and shortly after start, but after warm up it sounds weaker, but then randomly goes back to normal for a period of time before neutering itself again. i haven't hooked a fuel pressure gauge up to it yet, but it doesn't seem to be losing pressure as it fires first time every time, and i always let it prime completely before starting, then sometime let it sit while i drop the top. but i'll check it anyway. i don't like being one of those guys who asks for help and then shoots everything down without at least trying it. seems like it has gotten more timid, not louder. i'm leaning toward a plugged cat and am going to pull it off and inspect once the exhaust cools.

 

ok so i know it is NOT the catalytic convertor.....
One thing i did notice as i was unbolting it was one of the bolts holding the flange side on was loose.... like not even finger tight. based on the rust patterning it was definitely leaking, so i made sure when it went back on i tightened it good. ironically enough.... the sound improved.

i also discovered i believe my weird idle is caused by the trans itself. i decided that since it goes weird when i let of the gas to brake i would shift into neutral when slowing down and then back into drive as needed. it worked, no funky idle problems. so i'm guess new trans and sadly i lack the funds for a T56 conversion.

still no idea where the explosion sound came from though i have more the look into.

I'll try the other ideas during the week as i get time.

 

Tried it again today.... nope just kidding, no effect and in fact shifting into neutral actually stalled the car out.....

 

just putting my ideas in, ummm, i have a custom sidepipe exhaust installed with glasspacks and after install it would pop...very loudly (like shot gun loud) after driving round for a lil bit noise went away. however the noise came back, i was cruising down back country roads doin my thing, i went from 75mph and down shifted...then BANG. it was so loud and and abrupt i almost swerved off the road cuz i thought it was a bomb. i went directly home and looked under the car, NOTHING. then i drove it round for a mile, now i heard soft then louder popping and a metal clanging. i went back home and looked under once again, this time i checked my COLLECTORS (where the headers and exhaust system hook up) sure enough i had 2 bolts loose on driver side and 1 bolt loose on pass. side.

i would go check all bolts on your headers as well as COLLECTORS, any slight leak in your exhaust system (headers-to-block, COLLECTORS, cat, pipe itself) will create back fire (loud popping or gun shot noises)

 

Banging sound was definitely exhaust/fuel related. I heard the exact same thing while cruising at 70MPH. Main jets on the carb were too lean overheating the exhaust. I was cruising like this for a while.

Just at that moment my tach connection got loose and rpm went to zero. I panicked and immediately threw it into neutral. BANG! I guess it momentarily got rich and raw fuel ignited in hot exhaust pipes.

Pulled over, didn't see any damage, motor was idling along happily. Plugged tach wire back into dist and was on my way.

 

i take it your talking about the 92 vert.
if so then yes, the VSS is a needed signal.
off the top of my head fuel control, EGR, CCP, AIR injection, TCC, and idle are all affected by the VSS signal.
air injected into the exhaust at the wrong time can cause a back fire in the exhaust.
with no VSS signal the EGR and CCP won't work. the dash pot effect of higher than normal idle during coast down won't turn on which can cause intermittent dying.
the added RPM may also affect the trans during coast down.
the TCC won't lockup.
and the ECM will be stuck in cell 4.
im not certain, but i believe the ignition timing is also affected by the VSS signal.

 

Do you HAVE headers? If so that would make sense. I've had the exact same thing happen. Collector bolts loosened up (I use fancy pants torque-to-yield style locknuts on those things now) and as soon as I hopped off the gas to shift... BANG! Did it again, BANG! Rich mixture off throttle into hot pipes with a massive exhaust leak will cause these backfires/gunshot sounds. I would have characterized it more like a gunshot than a grenade, though.

 

well at least im not the only one who suggested headers/collectors. thanks for backin up my statements.

 

No headers, stock manifolds. but i'll check the bolts any way - Thanks for the suggestion.

yes this is on the 92 Vert - I'm leaning toward the VSS and the fact my speedometer died and shortly there after all this weirdness began. Going to order my new Drive (15 tooth) and Driven (42 Tooth) gears and see if that makes a difference.

 

Ok this post is a bit dead, but I figured in case someone else has a problem they can see the resolution to this as it was never answered.

Once I got the new gears it took 4 VSS's before it finally worked. The VSS's kept crapping out after a block. Things seemed great for a few months and then BANG out of the blue it happened again, but this time the speedo didn't die. Periodically I would get a repeat of the backfire.

I decided to have a custom chip burned that removed the EGR and moved the TCC Lockup to 55MPH. Again things seemed great, and though the code 32 light went away, it was replaced by a code 45 (running rich). I ohm tested the injectors and found I had two that were ohming out in the 7's and the other 6 were ohming in the 17's when warmed up. I then replaced all the injectors and replaced all the vacuum lines, and plennum gaskets. I also bypassed the throttle body heater lines. While replacing the injectors I discovered a valve cover gasket leak and my PCV grommet was dried up and leaking.

I decided to replace all those and do a tune up while at it. While doing the tune up I remembered what a huge pain in the **** it was with all the smog lines in the engine bay - so I removed them. While doing all of that I finally discovered the cause of my issues... drum roll please.... 3 of my 8 plug wires are/were chewed up on the back of the wire out of site. So they were arcing, causing the unburned gas to pool in the hollow cat, fooling the 02 sensor into thinking it was running rich, and then finally detonating. I imagine this will probably dramatically improve my cars performance and gas mileage. If I can get a week on a tank of gas I'll be happy..... Guess we'll see - But there is the answer to my Backfire from hell.