Exhaust popping
 

Well, I thought I had this thing licked but I guess not. If you've read any of these threads you'll know what I've done, but I've still got a popping noise from the exhaust.

http://67-72chevytrucks.com/vboard/s...d.php?t=370615
http://67-72chevytrucks.com/vboard/s...d.php?t=370264

The short story is just about everything has been replaced and I still have a little popping or spitting noise coming from the exhaust. It seems to only happen under acceleration, particularly under load (i.e. I can't reproduce it as easily sitting in the driveway). It also seems to get worse as it warms up.

Vacuum is 18 and steady, timing is 8 btdc, the entire ignition system has been replaced, the entire fuel/air delivery system has been replaced. The exhaust from the cat back has been replaced. The heat riser functions properly. The idle mixture was set via vacuum to 3 turns out on a newly rebuilt carb. The choke has been adjust to open fully when warm and close to about a 1/8" gap when cold. Compression is good and even all around.

It does idle a little rough. Not bad, but just a little miss every couple of revolutions. The engine supposedly has about 79k on it. It could be 179k, but I'd say that by the condition of the rest of the vehicle, 79k is not unfeasible. I don't know anything about the condition of the internals.

As far as power, it's hard to say. I've been driving an '02 Silverado for the last 5 years so I don't think I can give it a fair shake. I can run down the highway at 75 no problem and it doesn't seem to downshift much. It does feel a little sluggish accelerating, but again my frame of reference is probably off.

The only thing I haven't really checked (and what I'm leaning towards) is some kind of valve train issue. My local Chevy dealer (who I've done a lot of business with and generally trust) quoted me about $1,500 out the door for a complete top-end rebuild. Not sure if I'll get to that point, but is that a reasonable number? Short of that, what else can I check?

It's not a huge problem and I'm driving it every day on the interstate to and from work, so it's definitely getting worked, which is good. I'm just kind of anal as I know there is something wrong and that just needles at me.

Re: Exhaust popping
 

something in the valve train is the only thing left and what i thought of after you said you replaced the ignition system as i was thinking there someplace

Re: Exhaust popping
 

So, what exactly would cause this? A worn cam, broken or worn springs, valves out of adjustment, all of the above? The same shop said they would do the valve inspection/adjustment for $250 out the door. I know I could do it myself, but the time and mess involved makes me lean towards having them do it, as long as that's reasonable.

Re: Exhaust popping
 

I'm going to ask a stupid question. Did you replace the plug wires? How about your vacuum advance? I fought this problem with "newer" plug wires and that is what I had it narrowed down to after complete rebuild on the HEI and replacing the vac adv.. Truck didn't idle rough though. I'd double check everything before I did a top end rebuild.Pop your valve covers and check valve adjustment.

Re: Exhaust popping
 

Entire distributor (which came with a new vac adv), plugs, wires and coil have all been replaced and checked. The plugs are not fouling as far as I can tell, the wires are routed correctly and there is no obvious arcing anywhere.

Re: Exhaust popping
 

Take the belt off the smog pump (if it has one) and try it. I had a worn out 305 that would pop like crazy with that thing running, although it was mostly on deceleration.

Re: Exhaust popping
 

I'll give that a try. I still have the catalytic converter, though so, theoretically, the smog pump should still be needed (or at least useful).

Re: Exhaust popping
 

Did some more tinkering today. Finally figured up the MPG for this thing: 8.6 mpg. Not good. I also noticed that I can get it to pop and blow out a plume of black smoke while in park after it's good and warm.

So, obviously, I am getting a bunch of unburnt fuel in the exhaust due to it running rich (at least that seems right to me). So, where do I start troubleshooting this? I have a brand new rebuilt carb that I ordered online, but I guess I really have no idea how it is adjusted. Like I mentioned, the idle screws are about 3 turns out, but that should have very little to do with the mileage driving back and forth to work on the interstate, right?

Is there some way to adjust the other jets in this carb? Or, is there something else causing the rich condition? It's a stock Quadrajet M4MED.

Re: Exhaust popping
 

Forgot to mention:

Stock 350
4 bbl carb
31" tires
700R4 and I use the overdrive on the highway
3.42 gears

Shouldn't I be getting at least 12-14mpg? Heck, the specs for that year say 15.

Re: Exhaust popping
 

Made a little progress tonight. I set my timing using the vacuum guage rather than the specs under the hood (8 btdc). I advanced it to max vacuum and then backed it off about 1.5 inches from there, just to be on the safe side (I think the recommendation is 1 inch from max vacuum). It ended up around 14 btdc.

The popping is not completely gone but it definitely seems fainter and the acceleration and throttle response is WAY better. It actually feels like a truck now, not a schoolbus.

Now, I don't know if this is doing anything for the mileage, but it sure feels like an improvement. I'm going to drive it for a day or so and maybe advance it a little more again and see if it improves any more.

Does this sound at all consistent with the symptoms I was seeing? Is advancing the timing likely to improve the gas mileage?

Re: Exhaust popping
 

For $1500.00 (or less) you can get a new G.M. Goodwrench 350 engine!

Re: Exhaust popping
 

Looks like $2k for a 290HP GM crate 350. I guess less if you don't buy it directly from GM.

If it turns out to be something of that nature, that's probably the route I would go. It makes a lot more sense. However, I'm not sure the valvetrain is the problem, yet.

Re: Exhaust popping
 

G.M. will sell it wholesale to you also if you try and save on shipping too. Shop around town at local dealers for best price.

Re: Exhaust popping
 

Popping could be a burnt valve or rounded exhaust cam lobe.

Re: Exhaust popping
 

Well, so much for that. Same symptoms today. Spitting noise in the exhaust, feels like it's choking on the highway. I verified that the choke is fully open when warm. The acceleration is better due to the timing advance, but it still feels like it's working way too hard to stay up to speed.

So, I'm still either running rich or not getting a complete burn (or both). What else can I check? If it is a valve-train issue, are there any diagnostic tricks I can try short of pulling the covers? I have very steady vacuum on the gauge, right at 18".

Re: Exhaust popping
 

give the motor to me at buy that new 350 already ;)

Re: Exhaust popping
 

Are you getting any rotten egg smell from the exhaust? I had one with a partially clogged converter that left me stranded on Christmas eve in a snow storm. Turned out the pellets had broken loose and once plugged melted everything together and pop! When that happened, it blew the muffler out the back and into the car behind me.. The converter however was intact and completely blocking my exhaust.

Re: Exhaust popping
 

My truck was running very rough while back and I kept getting lots trash in fuel filter. I pulled tank and the sock had fallen off the pick-up and it was sucking lots trash in fuel lines. I replaced tank and sending unit and no more porblems.

Re: Exhaust popping
 

New convert, new muffler, new intermediate pipe

Re: Exhaust popping
 

Wouldn't that make it lean, though? Everything here points to it running rich. It's not really rough, it's just sluggish, uses a ton of fuel and causes popping in the exhaust, which appears to be unburnt fuel.

Re: Exhaust popping
 

wrong size jets? was it allways doing this after the new carb?

Re: Exhaust popping
 

That's what I'm wondering. I can't remember if it did it before b/c there were so many other problems (timing off, choke not working, major vacuum leaks). That's what it feels like, though. It's not rich at idle, I don't think, but it definitely seems like it at highway speeds.

Are the non-idle circuits adjustable in a Q-jet or do you just have to change out the jets?

Re: Exhaust popping
 

im not a expert on qjets (but i did stay in a holidayinn last night):lol:

Re: Exhaust popping
 

Well, adjusted the valves and put new gaskets on tonight. I also blocked off that damn EGR for now. It's really in the way when you're trying to work in there.

It seemed to run well when I test drove it, but I can't honestly say I noticed any difference from before. The spitting noise in the exhaust is still there after it warms up, but it seems fainter again. I may be fooling myself, though, so I'll drive it for a couple days and see what I think.

If this doesn't get rid of it, the only thing left is the carb. I emailed the guy I bought it from to ask about how it was rebuilt. We'll see if I get an answer.

I guess this isn't that big a deal, but it just keeps nagging me. I may have to move on to other things like replacing my u-joints and figuring out where the slop is in the front driveshaft that is causing this clunking noise when I'm in 4x4. It never ends.... :)

Re: Exhaust popping
 

Popping in the exhaust can also be caused by a leak in the exhaust system. A leak up-stream can allow fresh air to be sucked in, especially during decelleration, which can cause the popping

Re: Exhaust popping
 

Cool, I'll look around. The cat and muffler have been replaced, but everything in front of that is original (at least since I bought it). The spitting or popping is mostly on acceleration and under load, but I'll see if I can find any leaks.

Re: Exhaust popping
 

Might check your distributor to see if it is tight. I might turn just alittle and throw every thing out we had a 73 suburban that was doing about the same

Re: Exhaust popping
 

Have you checked your plugs?It may narrow the problem down to one cylinder.I'd be looking for one or more sooty plugs.You could also pull one v.c at a time and run it for a little bit looking for one or more rockers not moving as much as the others to rule out a bad cam.

Re: Exhaust popping
 

Checked the plugs twice, but it couldn't hurt to do again. Will probably try watching the valves. That sounds like a good next step.

Re: Exhaust popping
 

if its poping like a back fire through the exhaust its in your valve train like burt valves or your rocker adjustment is to loose witch could be your cam going away

and if you have a plug thats not burning fuel, once the fuel makes its way to the exhaust manifold it ignights the fuel mix and that will give you that poping sound
open up your gap on your plugs and see if it helps you can also use a spray bottle of water and spray your exhaust manifold at the port and see if one is burning colder then the others

food for thought
back fire through the intake likely ignition system
backfire through exhaust likely valves
new spark plugs dont mean their always good

hope this helps with figuring it out

Re: Exhaust popping
 

Thanks. I stuck the EGR back on because I was getting pinging and that seems to have gone away. So, now, I really need to try it out with the valves having been adjusted and with the EGR back on.

Re: Exhaust popping
 

Several others have mentioned the cam, I'm wondering if it's wiped a lobe myself. Just pull the valve covers and remove power to the HEI so it won't start. Have someone turn it over while you watch the rockers and see if you notice a lazy one.

Also, don't rule out the carb just yet. If you know of one you can borrow off a good running truck, bolt it on and try it out.

One other thing you can check by hand or with an infrared thermometer (if you have one or can borrow one/rent one) would be to start the engine dead cold and check the exhaust port temps either by hand or by infrared thermometer and see if you have one not warming up at the same rate of the others.

Re: Exhaust popping
 

Yeah, I probably shouldv'e thought of that when I had the covers off :)

Also a good idea. I have my old carb still. The choke was fried, but it still ran.

Good thinking, never thought of that.

Re: Exhaust popping
 

Sounds like the same problem i had on a small block 400, Under hard acceleration it would pop through the exhaust. It had a bad cam,3 round off exhaust lobes. A new cam and lifters fixed the problem. I also did what was suggested earlier to verify. Take valve covers off and watch rocker arms while rotating engine, The bad ones will not move as much.

Hope this helps
87shortcrewcab

Re: Exhaust popping
 

Im not an expert like others on here so i can only speak from experience. I would double check the plugs as mentioned earlier. Thats what solved my issue with the popping like yours.

Re: Exhaust popping
 

try checking the deverter valve diaphram if equiped. if defective, replace it, fixed my popping issue.

Re: Exhaust popping
 

had this issue is a older 350 someone put a nylon gear (found in 305s) in there and it had a crack......didn't see if you had an air pump.......no leaks and working?

Re: Exhaust popping
 

I do have an air pump and as far as I know it's working. Where's the diverter valve and how do I check it? I see a little canister mounted front-left (passenger side) that the AIR system is hooked to, is that it?

Re: Exhaust popping
 

That's starting to sound more and more likely at this point. A couple questions, though. Would that make the fuel mileage horrible (I'm getting around 8mpg)? And, my issue is only happening under load (like going up a hill), would that be consistent with a worn cam?

Re: Exhaust popping
 

the deverter valve is just above the air pump. its job is to divert air flow to your air intake on deceleration and heavy exceleration.and sends air flow to the exaust while at cruse and idle.

you should have three hoses leading from it. one to the exaust,one to the intake and one to the air pump.
checking it is not bad. take a vacume reading at at max apply it to the valve diaphram and see if you are losing
vacume. if you lose vacume you have a bad valve.make sure all other air leakes in the exaust are taken care of before replacing
the valve.