Backfiring

[86esprits3]

I warmed the engine up nicely and took the car for a spin today. Keep in mind that I just got this car rebuilt and running. I am sorting it out slowly. Timing is okay-vacuum lines are all new. Idles great. But when I took it for a ride, I can get it to backfire pretty regularly through the exhaust when I ease off the accelerator and the engine is around 2800-3500 rpm. I get a pretty good pop. Sometimes it is alot louder than at other times.

On a side note, I removed the fuel injectors and stuck them in clear bottles. I bypassed the relays to get the fuel pumps on. When I pushed down on the mass air flow damper, I got a really nice fan spray pattern out of all injectors. I then turned the fuel mixture screw on the mass air sensor to enrichen the mixture until I saw drips form on the injectors, then turned it back lean 1/2 turn. This is how the manual states to start out the adjustment when not known if someone tampered with it. I pushed down again and let the injectors spew out a good amount of gas to all 4 bottles. Then I compared the volumes-all the same-a okay.

I had the same backfiring before I adjusted it, and I counted the turns, and arrived at pretty much the same place I started. I believe backfiring is from a mixture that is too rich.

So what do you guys think? My 10 minute ride gave me about 6 good backfires-I dont think this is a good thing!

thanks

Mark

Edited October 16, 2007 by markd

[iainskea]

Mark,

I realise you are focussed on the fuel injectors, but have you considered an exhaust air leak? This is the most common cause for backfires - mostly the triple clamp, or loose manifold nuts. There are some other posts on this I think...

Besides, a good backfire just make more people stare at your car - good result really!

Iain

[Splashout]

bear in mind that if you are accelerating with any vigor towars 3k revs then you will get some over run affect. Whereby the engine is getting more fuel than its burning and this in turn hits the hot exhaust and ignites giving the pops and crackles that we all like

This is more prevelent on carbed cars by the nature of carbs, but may be something that could be happening however on mine I need to really hit 4k revs and let off to get any sizable bangs.

[cweeden]

Hi Mark,

When I had my '87 Turbo HC it developed a backfire on the over run but I didn't think too much of it (I quite enjoyed it to be honest). However at the next service they ran a compression test and found it to be way down on the expected compression. They commented that they were surprised it was running as well as it was. It turned out that the exhaust valves needed replacing.

I don't expect this to be the case on a recently re-built engine however it may be something to bear in mind and maybe something has not quite been lapped in properly.

I would be more inclined to agree with Ian's comments about an air leak in the exhaust system somewhere.

A non standard exhaust that breaths more freely may also backfire more than standard. Have you changed the exhaust during the rebuild?

cheers

-Chris

[indiaskr]

Hello Mark,

I have a '86 HCPI model. When I first bought the car, the previous owner claimed that the engine was recently rebuilt. However, when I received the car, it started backfiring on deceleration. It turned out that the valve guides were the wrong ones during rebuilt. My mechanic put in new valves and proper guides. Never backfired again. I do not know if this is your case, but here is another data point.

[WayneB]

Mine pops and gets flames out the exhaust on the overun (but it doesnt backfire like a shotgun blast)

[86esprits3]

Mine pops and gets flames out the exhaust on the overun (but it doesnt backfire like a shotgun blast)

Not quite a shotgun blast, but I bet the neighbors have no problem hearing it inside their home!

[WayneB]

Mark, I'm wondering If your mixture is actually set too lean (which will cause a backfire), try richening it a tad and see If it improves things.

[86esprits3]

Mark, I'm wondering If your mixture is actually set too lean (which will cause a backfire), try richening it a tad and see If it improves things.

Okay Wayne-I will try it!

I am having a ball actually driving this car. I need to get it on the highway. I am getting it inspected this weekend, then the cats coming off. Inspections are visual for this model year in my county, so it should be easy to pass!

[WayneB]

Mark,

I thought you had already removed your busted up catylitic converter, a Knackered Monalyth could definately cause a backfire !

[86esprits3]

Mark,

I thought you had already removed your busted up catylitic converter, a Knackered Monalyth could definately cause a backfire !

I gutted the cat out, but the empty can is still on it. I guess that could be the culprit. I am keeping it on for the inspection, then its off.

[f1karting]

Air fuel mixture, your running lean somewhere...