Thoughts on backfire?
 

As many of you know, I just went from stock catted manifolds and straight pipes to coated OBX headers (no cats) and Corsa. Sent the PROM to Marc and he removed the DFCO. Yet, this thing is backfiring like an SOB!!! It backfires loudly on nearly every upshift. Frigging embarrassing!

Bill Boudreau data-scanned the car and it is running unusually lean on deceleration. Marc says this is common with FIC injectors and says Accel are better about this. I spent enough cash installing the new FIC and I'm not changing to Accel just to "see what happens". Marc says he can add fuel on decel but isn't sure if this will fix it.

What am I missing? Do I have an exhaust leak? And if so, can an exhaust leak really cause such raucous backfiring? Can I really be the ONLY guy running headers and Corsa without cats whose mufflers are about to blow clean off the car?

TIA!

Re: Thoughts on backfire?
 

two things that helped mine-
1) disable the air injection when running catless (I kept my tubes). The air injection leans the post combustion pipe mixture.
2) add catalysts to muffle the pop.

I have the FIC Lucas type injectors. Turning off DFCO worked for me, but there is the possibility of a backfire during the warm up event when the air injection pump is still running...otherwise only goes off once in a blue moon. Cats help a lot.

Re: Thoughts on backfire?
 

The consensus says FIC injectors are not very good. I am about to replace mine with the accell 150821 units. I do not have any backfiring but I am in need of new injectors.

When I purchased the car last year the prior owner had FIC injectors installed. I guess they didn't last long with the ethanol in todays fuel.

Re: Thoughts on backfire?
 

I would absolutely make certain there are no exhaust leaks. Decel produces high vacuum and improper exhaust system sealing will suck fresh air into the exhaust. And it does not need to be at the collector. Could be further back and it would still cause a popping.

A few questions:

1. Did Bill note if the BLMs are leaner in one bank versus the other during decel?

2. The datalog should show the injector pulse width. Does it look like injectors are shutting off?

3. What about the O2 sensor voltages on both banks? What are they during decel? Are they different from each other?

4. Can you tell where in the exhaust the popping is happening? At the header or further downstream?

Re: Thoughts on backfire?
 

The backfiring is definitely happening at the mufflers. Marc mentioned the injectors and asked about the O2 sensors, as well. But the more I think about this, the more it seems like it must be exhaust leaks. 3 weeks ago, the car had straight pipes (except for cats) and was EXTREMELY loud. Also, the DFCO had not been disabled at that point, yet there was virtually no backfire. The injectors were changed in February, so they're not the problem. Bill drove the car thoroughly several times after install and probably would have noted loud backfiring if it was happening then, which tells me the situation is worsening over time. I'm thinking exhaust clamps are loosening with heat cycling. What do you think?

Re: Thoughts on backfire?
 

Chris,

That's an important observation. Leak is the first thing I would suspect and inspect for. Decel would cause fresh air to be drawn into hot exhaust. Would also explain lean mixture. Which side is it or is it both? Yes I've had clamps stretch.
First time I ran into this was after putting MagnaFlow muffs on my 84 Xfire. Doing some tuning, and I was constantly needing to add fuel in the "decel region" of the VE table. That's around 20-30kPa. Until one day I walked up behind the car and noticed the left muffler hanging low. Fixed that, and then I needed to take a ton of fuel out. So the O2 sensor can be affected even by a leak quite a bit downstream of it basically making the situation worse.

Re: Thoughts on backfire?
 

I had a few noticable exhaust leaks where the collector and the exhaust meet up. I used red high temp silicone RTV sealant to fix it.

Re: Thoughts on backfire?
 

Thanks Dom. Not sure on which side, or whether it's isolated to one side or the other; I wasn't able to discern that on my drive. I'll just need to get under there and start wrenching to see if that works.

Re: Thoughts on backfire?
 

I haven't heard this consensus....

Re: Thoughts on backfire?
 

I'm with Kevin, always had good luck with them!

At least a half dozen sets installed over the years.

Brad

Re: Thoughts on backfire?
 

That's what I have been told up here in FBI land.

I had a 1992 Nissan Maxima and the injectors on that car were well know for failing due to ethanol in the fuel.

If I knew about the failing injectors on the LT-5 before I purchased I would have purchased a 1993 or later model year.

Re: Thoughts on backfire?
 

no zr-1 is immune from failing injectors

Re: Thoughts on backfire?
 

I suppose you are correct but I have never heard of a failing injector on a 1993 to 1995 LT-5.

Re: Thoughts on backfire?
 

Re: Thoughts on backfire?
 

Headers, no catalysts, and a Corsa (no)catback is basically straight pipes with a couple of chambers for standing wave acoustical tricks.

I thought the Corsa was too loud with the factory manifolds and factory cats- I doubt there is much to be done about the intense popping. Usually conventional mufflers and cats muffle the lean pops to a common burble.

Re: Thoughts on backfire?
 

you must have missed the work we did on lance millers yellow car.

Re: Thoughts on backfire?
 

Sure did!

Re: Thoughts on backfire?
 

I have had my Accel injectors since 08. Never a hiccup.

Re: Thoughts on backfire?
 

I've never heard ANYone say ANYthing that could be construed as ANYthing but +, about Jon, or FIC. I've talked with him, AND his help, and seen his address to customers on forum boards, with issues that customer THOUGHT would be fixed by new INJ's install, but were failed customer diagnosis, and I don't know too many business entities that are better, when it comes to CS.

If you had a bad experience, I think you should contact him, and allow him to address the issue. And he's done so in the past, ON FORUM BOARDS FOR ALL TO SEE.

============================

KS; looks like you might have a couple of issues here; a little FSM reading (never fun, but highly informative) might lead to a common root issue solution, such as a shaky ground on an important / multi-ground connector...

Re: Thoughts on backfire?
 

Will an exhaust leak cause backfire, when he's got a good burn in the cylinders?

If so, what burns? HC's? Will HC's light up SES?

Got SES there KS?

Re: Thoughts on backfire?
 

I don't want to turn this thread into battle of the fuel injector manufactures/retailers.

Bottom line is I am going with the Accell units. There are at least two well respected LT-5 tuners not saying the best things about them. One is mentioned in this thread.

I am holding the receipt from FIC, purchase date 06/07/2010 "delphi new 92 zr1 2nd day." I did not technically have a bad experience with anyone at FIC. I just have a product of theirs that I am not happy with.

This is not a job I want to do twice, let alone once.

Later!

Re: Thoughts on backfire?
 

No sir, nothing out of the ordinary, other than the backfire. When I'm shifting somewhat slowly, it actually backfires when I depress the clutch during full-throttle acceleration runs. Marc says he's seen maybe 1 car in the past 15 years backfire at such high RPM.

2 changes occurred: the new exhaust and Marc reprogrammed the PROM to add fuel and disable the DFCO. I'm baffled and even Marc doesn't really have an answer.

Start watching the "ZR-1s FOR SALE" section, guys...

Re: Thoughts on backfire?
 

Same as my RC injectors since 07 never a hiccup

Re: Thoughts on backfire?
 

Karl,

I have a 94 and my injectors went bad.

Re: Thoughts on backfire?
 

When and what did you replace them with?

Re: Thoughts on backfire?
 

Looks like an injector debate thread is in order.

OP probably isn't too concerned with that debate ATM.

==========================================
KS; you do static FP tests? Leakdown time?

--------------------------------------------------------------------------

The fuse circuit overload is a concrete clue. Maybe start there, to that thread...

Re: Thoughts on backfire?
 

Thanks Schrade, you're spot-on there! Frankly, I'm amazed with the track these threads often take. Anybody remember the exercise in 3rd grade where the teacher whispers to the 1st kid a topic and each kid then whispers to the next his take on said topic? What comes out of the mouth of the 30th kid is a million miles from what the teacher originally said. Good example: go check out my 4.10 gear thread. I'm still sorry I posted that thing.

Nope, the tests you mention are Greek to me. I let Bill handle all that, as I really just want to drive and enjoy the car. The fuse deal isn't related, though; that started when I got the engine compartment wet on Tuesday.

Re: Thoughts on backfire?
 

header bolts loose?, manufacture of header gasket?
You were not enjoying the no cat smell, maybe cats will eliminate back fire?

Re: Thoughts on backfire?
 

Hey Phil, yeah no indication that it's an exhaust leak at this point. Adding fuel or cats would only mask the problem; there's something else that's causing it. Marc suggested injectors and I just talked to Bill and he did, too. But my point is that the injectors seemed fine a few weeks ago, there was no backfiring with my old straight-pipe exhaust system. This is very frustrating, to say the least.

Re: Thoughts on backfire?
 

Bingo you will mask the issue.
If all is good,meaning you don't have other issues, I'm with Marc about injectors.

Here's an idea buy/install Accel injectors if it fixes your issue then return the FIC's for a refund if it does not fix the issue I'm sure Karl will buy them from you, at a discounted price :-D jk.
Let us know the outcome.

I like to do injectors once since it's so involved to change injectors in our Z's plus gaskets.

Myself and all the fast Z's here in FBI land run Accels for the past 5-10 years no issues and they all run like Bat out of H@ll :cheers:

I don't see battle of posts just exchanging ideas.
Agree to Disagree
Pete

Re: Thoughts on backfire?
 

They were replaced in April 2009 with FIC rebuilt (cleaned?) Bosch units. A couple of days ago I suspected leaking injector but seemed fine yesterday. We shall see...

Re: Thoughts on backfire?
 

Chris, I can see adding some fuel in specific areas helping, but I dont see DFCO changes helping a normal driving upshift "backfire'. DFCO could be an issue if you were having an ingear off idle coast down backfire.

Is you exhaust 2 seperate pipes from the headers back?
No sort of cross over correct?

You are accelerating at WOT, then shift time comes, does it pop when you press the clutch, or pop when you let off the throttle.

You get a pop during slow shifting, this would assume a closed throttle, but if you get a pop as the engine is unloaded while still in WOT fueling mode, thats another.

Sounds like a powershift test is in order.

Any reports of flaming out the pipes during shifts or if you wind her up in 2nd gear and let off the throttle without touching the clutch and let the engine slow you down. If there is substantial nastyness happening during teh pulldown, couldbe pulling atmosphere when teh entire t-body back plenum is in extreme vacuum conditions, injectors and their sealing could have effects here as well.

If adding a touch of fuel via calibration isnt masking anything, its correcting an instability that you introduced from a non-OEM exhaust.

I dont see injectors magically failing before during a header install, esp. non OEM 90-92 injectors, but weirder things have happened.

Sorry to be so wordy, just trying to talk it out.

Not trying to put anyone down, but when you have a customer asking Why? Why? Why? And you dont have an answer supported by your own diagnostics, it's easier to start down the customer down the "common issues-tree" to try an appease said customer.
Sometimes the answer should be "I dont know?" bring it in and I'll go through it", this is human nature to some, again not directed at anyone, of necessarily even applies to your issues.
I've had to do this with people personally, and some people get nasty when you say "I dont know what your issue is, but after I do a diagnosis I'll have a better idea" When in my head I'm sometimes thinking "Sorry I cant fix your vehicle over the phone, I'm not a mind reader".

Re: Thoughts on backfire?
 

How soon after the header install was the calibration changed or was it concurrent? Any chance we can see Bill's datalog?

Re: Thoughts on backfire?
 

Gentlemen,

Thanks so much for all the input; it is GREATLY appreciated!

I think the mystery may be solved. I just heard from Marc. He double-checked my file and discovered that anti-backfire did not get enabled on my calibration. I'll ship it to him today and will hopefully be a little less noisy by next weekend.

Thanks again! Stay tuned...

Re: Thoughts on backfire?
 

I thought you liked it loud, hopefully that works

Re: Thoughts on backfire?
 

If you think you have exhaust leak causing backfire (personally, I DON'T, because exhaust shouldn't burn, unless there's unburned fuel in it, but I never got paid for wrenchin'), but anyway, spray the slip-fits with WD, then loosen them, and re-tighten them.

I did that with mine after a few days final torque-down, just to make sure that they were seated. Maybe do it while they're still warm.

Some FP snappics here:

http://www.zr1.net/forum/showthread.php?t=23485



FP test is just like checking air in your tire.

Re: Thoughts on backfire?
 

My experience has been that slip fits do exactly that. They slip. Mine are tack welded because they have a tendency to twist from the torquing of the motor.

Re: Thoughts on backfire?
 

Should have the PROM back from Marc by tomorrow. If the "anti-backfire" feature lives up to its name, then all will be well in my garage.

As long as I can fix my fuse issue, anyway.

Re: Thoughts on backfire?
 

Sounds good; find that stray ground on the fuse circuit?

That should be an easy fix...

Re: Thoughts on backfire?
 

Not yet. I'm sure I just got something wet under the hood. Dave suggested that I remove the battery and use a hair dryer on the bundle of wires that go through the firewall under there, which I intend to do but haven't yet. Just haven't made it a priority yet since the car has no PROM in it anyway.