How to solve an exhuast backfire?

[Polyphemus287]

350 runner,

I bought the truck off a gentleman who drove it probably twice a month. When I got it had a little over a quarter tank of whatever gas he ran. After buying it I took it to gas station and filled it up with about 35 dollars of premium 93 octane.

350runner and 87scotty,
I didn't have a chance to play with the timing yet as it has been raining here and I don't have a covered garage. I will probably play with it first thing in the morning when the sky clears.

Also, just out of curiosity, could a broken vacuum advance cause this problem/ affect it?

 

[crazy4offroad]

Bad distributor vac advance could definitely cause it. You say you replaced vac lines, did you replace any that weren't there before? Are you sure they are routed correctly? Are they now attached to a dashpot or vac switch that is malfunctioning/non-functioning and causing the backfire? May want to get a vac pump to test things like the distributor advance and dashpots and whatnot. May also want to get a vac gauge, it will help determine if you have a vac leak, bad valves, etc. May also be time to tune up the distributor with a new pickup coil, ignition control module, etc.

 

[Polyphemus287]

So here is the latest on the truck:

Went out to see what I could do in terms of timing, but before I did I reinstalled the air cleaner. I had the air cleaner removed during the process of the tune up, and checking for a vacuum leaks. Upon reinstalling it and the vacuum lines that go to it, I started the truck which proceeded to idle very rough from a cold start (lots of shaking etc.)
This is strange because when I removed the air cleaner it idled noticeably better with less shaking and so on, like it did when I first completed the tune up. Puzzled by this I decided not to fool with the timing at this point, I will probably mess with it tomorrow. Also, the air cleaner situation led to no change in the backfire.

What do you guys think about this rough running as a result of reinstalling the air cleaner?

crazy4offroad,

I did not reattach any lines that were not previously installed. The truck at some point during its life lost the vacuum diagram under its hood, and my repair manual lacks a schematic of it. I'd like to reattach all of them, but I have no idea where they go.

Are there any vac switches that could cause this issue in particular? I'm not particularly familiar with how to use a vacuum pump, but I'd give it a go if necessary.

 

[crazy4offroad]

Sounds like the air filter is restricting airflow. OR whatever vac lines you connected to the airbox are connected to something that is malfunctioning. Unless what you connected was the larger crankcase vent, in which case the baffle could be plugged if there's much blowby. You may also want to check the PCV valve. I'm sure someone here could come up with the schematic for you for the vac routing. As far as the vac pump, it just has a gauge and a handle similar to a caulking gun, you squeeze it several times and it pulls a vacuum on whatever you connect it to, like the distributor vac advance. You pull 20" or so of vacuum and see if it bleeds off. If it does the rubber diaphragm inside is dry rotted. You can also connect it to vac switches and use a couple pieces of clean vac hose to check if when switched the vacuum is routed to different ports like it is supposed to. Some vac pumps you can connect to an open port on a running engine and tell if there is a vacuum leak if it doesn't hold enough vacuum. A shaky needle may mean a sticking intake or exhaust valve. A steady rise/fall could be from one cylinder having bad rings, etc. A regular vacuum gauge like you buy in the tools section of a parts place has a chart that tells you what each reading means but you can also google "vacuum gauge chart" and check the images, it will give you a chart similar to this..

You must be registered for see images

Hope something here helps.

 

[chengny]

To my understanding a backfire out of the exhaust is caused by unburnt fuel flowing through the exhaust and igniting. My initial impression is that my tune up somehow resulted in the engine gaining more fuel flow than before.

Yep that's half of it. The other half of the equation is that there is a sufficient supply of oxygen leakage into the exhaust system to cause that secondary combustion. I'm thinking that prior to all the good work you did - tightening up air leakage into your intake system (and possibly even restoring the fuel supply to normal with the new pump, filter etc.), the PO was running excessively lean. As you noted, the plug tips were noticeably covered with white ash.

So now you have a higher F/A ratio than before. It may even be correct - don't assume carburetor issues yet. But the case may be that someone "tuned" the carb in an effort to enrich the F/A ratio.


What will help the most - as far as the backfiring - is for you to do a thorough inspection of the exhaust system - especially in the areas of the manifolds. Repair what you find. Whatever you can do to reduce the ingress of air into the exhaust piping will help ease the popping you are experiencing upon decel.

An excellent way to identify exhaust leaks is to inject a small stream of MMO (Marvel Mystery Oil) down the carb throat - with the engine idling fast. It produces a highly visible white smoke that will flow out of the smallest leaks.

 

[350runner]

Chengy you are correct, though a pop not backfire heard during deceleration means the engine is running lean. It's a light pop not a loud gun sounding backfire. Either way it's not harming the engine as the engine is running on its most tolerable high a/f ratio. Probably gets good gas mileage as is just little low on power. But hey it's a6 cylinder...

Sent from the dust in front of you!

 

[350runner]

Poly287,
Can you post a picture of the front your carb? Mainly the area where the a/f mixture screws would be?

Sent from the dust in front of you!

 

[Polyphemus287]

Thanks to all for the suggestions,

Unfortunately, I was at work the rest of the day and unable to tinker with the truck.

Crazy4offroad,
I will pick up a vacuum gauge first thing in the morning and run a test. I'l post my results here as soon as it is done. Also, thanks for explaining the vacuum pump. If my vacuum tester leads me nowhere I will try the pump next.

Chengny,
I never thought about an exhaust leak as a problem. Can't believe I didn't think of that. However, when buying the truck I did plug my hand over the exhaust with no hissing apparent. This was done from a cold start as well. I will look the exhaust over tomorrow morning, and try the mystery marvel oil trick as well. Also, I hope no one attempted to tune it, as to my understanding it is non adjustable.

350runner,
According to my manual these screws are preset by factory, and shouldn't be adjusted unless necessary. Essentially the screws are underneath a wire mesh that needs to be removed to adjust. However, I will post a picture first thing in the morning.

 

[Polyphemus287]

here are my schematics in lieu of a real picture. I will post an on car photo tomorrow.

 

[Polyphemus287]

Ok so ran a few tests, and checked a few things out.

The readings for the vacuum test I preformed with a warm engine:

At idle the motor read just between 16.5 & 17 psi.
There was a slight vibration in the vacuum gauge at idle, and by slight I mean .3 of psi +/- in either direction.

When i gave it some throttle the lowest the throttle dropped was 5 psi. I hit the throttle several times and on average it dropped to between 7-10 psi. I am not sure how to interpret these readings as to me they down clearly point to issues listed in the schematic above.

I also went back through the exhaust, and found some sort of leak I believe. I think have narrowed it down between the donuts that mate the header to the down pipe, or a leak at the manifold itself.

I didn't get a chance to pick up the Marvel oil today but i will probably give that a go in the am.

Any suggestions on these readings?

Edit: I re-ran the vacuum test tonight. I did it from a different manifold port and got the same results as before, except this time I needle dropped to zero under throttle. I guess that means my rings are worn out?

 

[87scotty]

Idk that sounds pretty normal to me at wot

 

[SlickGTP]

My truck from high school started backfiring when the exhaust doughnuts failed at the manifold to exhaust piping.

The exhaust leak wasn't real noticeable, but the backfire was.

Just make sure if you replace those, you tighten the bolts back up evenly. I didn't/couldn't and the new doughnut ended up prematurely failing leaving me back at square one.

 

[Polyphemus287]

Sorry for lack of updates, but I have been busy of late.

Yesterday, I took the truck to get a VA inspection, because well I had to. Confirming my suspicions the shop failed me for an exhaust leak. However, they were not sure either as to were it was leaking. The best I could get out of them was it was probably one of the donuts, or it was the coming from the header.

This morning I proceeded to try chengny's trick with the MMO. I did this from a cold start hoping it would make any leaks more pronounced. I gave it about 1/4 throttle probably reving to around 2500 rpms, and not a single puff of smoke came out. Also, I had a helper plug the exhaust while I poured the MMO in the carb.

I can see both donuts from the top of the engine bay, and neither has any visible cracks or black soot on them. The header on this thing is pretty impossible to inspect short of removing it, which I would like to avoid since it is mated to the intake. Basically, it would require me to remove everything in the proximity of the drivers side of the engine.

So my conundrum is this. I need to get this on the road, so I'd like to slap two new donuts in it and see what happens, but what if that doesn't solve it and I have to remove it twice them when I take off the header? Can donuts be reused? My inclination is to just replace the manifold gaskets all together and be done with it. Is there any other method that would better diagnose this exhaust leak?

 

[rich weyand]

You might try this:
http://www.instructables.com/id/Find-an-exhaust-leak-without-burning-your-hands/

 

[Polyphemus287]

Rich,

That is an interesting method, but unfortunately I don't own a sweet dyson vacuum like that guy. In the next day or so I will take the truck to a shop with a smoke machine and see what happens.

I will post back as soon as I get some results.