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HowardRoark

V8 Engine Backfire / Popping

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Hello all,

I recently had my 918 engine completely rebuilt, and it has driven very well for about 4,000 miles so far, but it has started to backfire a bit when decelerating.  It normally does it when I take my foot off of the throttle.  It is quite loud and sounds like the crack of a gunshot.  It never backfires when accelerating.  

When accelerating in 2nd gear, there sometimes is a very slight hesitation in acceleration, but not sure if these two issues are related.   

Also, when idling, it sounds smooth, but there is sometimes a very faint “poof” sound.  Not a distinct pop, but very faint and intermittent.

I know some owners will say that the backfire/ popping sounds are normal, but I do not think it is normal, and I think it can be fixed.   Any suggestions on what to check or change?

cheers!

 

8E20935F-1D64-4EE4-86CA-455A8CBD6DB1.jpeg

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Upgrade today to remove Google ads and support TLF.

Had the exact same symptoms and was getting a bit worried that something bad was going wrong - turned out to be a leaky exhaust joint.

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A few years back on my 2.2 the exhaust valves were worn and caused the popping and banging. Similarly on an old GSXR750 a small hole in the exhaust manifold created some impressive back firing too.

Check the manifold studs the original Lotus studs have a habit of shearing off and could be causing a leak at the manifold(s). I replace mine with ARP rolled studs when I had the chance.

cheers

-Chris

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Hi all, my tech has checked the exhaust and there doesn’t appear to be any leaks.

Question  -  would a faulty Waste Gate valve cause some backfiring, by letting too much gas back in the exhaust??  I am wondering if it could help with the issue?  As I understand it, the WG prevents too much boost going back in when you let off of the throttle, and my car does backfire when I do let off of the throttle.  Are these related?

cheers

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Hi Matt,

Not sure if you have sorted this already or not but I happened to stumble over it again and spotted what looks like a slight misunderstanding of what the waste gate is doing. I think you are confusing it with a dump valve (or blow off valve).

The turbo is driven by the exhaust gasses and once the desired amount of boost is reached the waste gate valve opens to let exhaust gases bypass the turbo and exit via the exhaust, it cannot let too much gas into the exhaust, this is just exhaust gas the more the merrier.

A dump valve or blow off valve is not fitted to the Esprit as standard, if you add one it would sit between the turbo charger and the throttle body (or maybe even the intake port itself ??) and when the throttle is closed the valve opens to release the boosted inlet charge to the atmosphere or sometimes into the air intake on the other side of turbo. This prevents a pressurised air from pushing back on the turbo compressor and stalling the compressor wheel.

It is possible I suppose that the waste gate valve on the turbo might be leaking via its actuator arm but I'm not sure that's common at all.

cheers

-Chris

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Idk if this will help but this is what mine is/was doing on the drive home. the left exhaust is the one that's doing all the popping at idle. 

compression check showed just  a slight drop on one of the left cylinders. Dont think it has anything to do with turbo. I believe excess gas is being dumped out of an injector or there is a bad coilpack which would explain the intermittent hesitation under load on hot days. 

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Matt sorry no help with your issue but what is your garage floor made of? Can’t be tiles otherwise you must have had the worlds strongest tiler. I really like it and as building a garage I am interested 

thanks in advance 

Dan

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Looks like textured and polished Concrete Dan,  Lucky Americans have bben doing it for years  !  

V8 backfire i would say is over fueling ! 


A

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15 hours ago, v8GTmac1 said:

Idk if this will help but this is what mine is/was doing on the drive home. the left exhaust is the one that's doing all the popping at idle. 

compression check showed just  a slight drop on one of the left cylinders. Dont think it has anything to do with turbo. I believe excess gas is being dumped out of an injector or there is a bad coilpack which would explain the intermittent hesitation under load on hot days. 

Popping on idle can indicate a cracked piston. An inspection camera will allow you to check this. 

Lamda sensors are also known to cause popping as well


Only here once

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A simpler cause could be a faulty O2 sensor(s). Do you have a OBDII? if so check the operation of the O2 sensors closed and open loop as had a similar flame event which was traced to a faulty o2 connection which was frustratingly intermittently loose causing over fuelling. The consequence was back fire on lift off also causing flames out the back, (driving during the day so not immediate apparent to me), however there was a hesitation on acceleration, dry to nurse the car back now with the EML I got the car recovered.  I replaced the connection and needed a new cat but right as rain now. 

PS I've always had popping on my car

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11 hours ago, Barrykearley said:

Popping on idle can indicate a cracked piston. An inspection camera will allow you to check this. 

Lamda sensors are also known to cause popping as well

the 02 sensor could very well be it. Maybe explains why i'm reading lean fuel trims on that bank.....I wont know again till it's a runner later this year. 

Edited by v8GTmac1

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Also consider air leaks from the inlet manifold and exhaust manifold. This can also lead to slightly rough running and an air leak will lead to a weak mixture.

Exhaust manifold studs are a known weak point so have an inspection in that area.

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f7ea5c25cfd740a1a47ec4c168803b76.jpeg

turned out to be true in my case.  2 missing nuts, 1 sheered off stud. 

Edited by v8GTmac1
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Seriously ???  the car was sold in that condition with the manifold simply not secured at one end?  With all the oil on the manifold, it looks like the nuts were soaked for removal.


1996 Esprit V8, 1998 Esprit V8 GT, 1999 Esprit S350 #002 (Esprit GT1 replica project), 1996 Esprit V8 GT1 (chassis 114-001), 1992 Lotus Omega (927E), 1999 Esprit V8SE, 1999 Esprit S350 #032, 1995 Esprit S4s, 1999 Esprit V8 GT (ex-5th Gear project), 1999 Esprit V8SE ('02 rear)

1999 S350 #002 Esprit GT1 replica

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Engine out job unfortunately and makes sure you use replacement stainless steel lock tabs. Be careful when removing the broken stud ( if broken below the casting surface) as these are a blind hole and you need to watch you dont drill into the head casting otherwise you will create another problem. This if it occurs can be solved but better to be aware of the potential issues before you start.

The low torque rating for these stud nuts which I recall is something like 6 - 10 ft/lbs, is to allow for thermal expansion and prevent the manifolds cracking. The manifolds expand and contract when they get hot ......very hot when boosting .......and then cooling after engine stopped.

Quite common for exhaust manifold bolts/nuts to work loose on any engine and the Esprit tends to suffer from this quite a lot and more so than any of the numerous V8 engines I have built and worked on road or track.  In fact I probably hear of one/two type similar problems per week with rough running some of which can be attributed to sensors, but also head gasket failure and air leaks from the inlet and exhaust manifolds.

Good Luck

 

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What if I told you guys my popping could be attributed to a simple ignition misfire?

Chain of events otherwise known as "the demise of my ignition system"

ECU repaired previously for unknown reason

bought and drove 350 miles home just cruising, shooting a flame on the left when on the throttle

noticed it was very under powered and popping on the left at idle for 2 weeks before teardown

15mo. later engine is rebuilt and I put maybe 4 miles so far.........

during the 3 or 4 mile adventure it started to progressivly misfire badly under any kind of boost only reaching 5psi

shut it down and restared it several times, gave it a wash, yadayada, it's starts to misfire at idle  

while chasing down oil leaks next weekend the misfire got worse at idle and was barely running

made sure the spark plugs were not wet from the wash

started pulling on the ignition wires 1 at a time to see which cylinder was missing and I found 6 & 7 where not firing a spark(You can also find with a timing light clamped onto the wires)

6 & 7 are paired together in 1 coilpack and fired together (wasted spark)

I priced out new coilpacks/gaskets etc 

meanwhile I decided to PinPoint exactly where the fault was and where I was going to have to spend my money

I pulled the ECM out of it's location, opened up the connector and exposed all the wires, found all 4 coil wires 29 + 49 , 32 + 52

I bench tested all the coilpacks with a home made IGBT ignitor and a 5v power supply, and found that the coils DID actually fire somewhat

Next test was to cut and swap the LEFT and RIGHT output wires from the ECU

              and "WITHOUT" the FUEL system engaged

              connect a TIMING Light to the ignition wires that were dead (not firing)

              crank the engine with KEY and found that the problem "MOVED" to the right side!

              This told me the problem was actually in the ECU (not triggering the coilpacks on 6 &7  PIn 52 at the ecm connector)

What did I do?   I opened up the ECU and found the IGBT in the board, DEsoldered It, bench tested it with a simple LED and found that it was not turning ON/OFF, it would stay on for a few sec and slowly die away. 

              So I replaced the 14N40FVL transistor for a BIP373 fully soldered it to the board, put it back together, back on the car and guess what?

              I went from having 6 cylinders operating to having 7 out of the 8 and even then the Popping was gone!  

              WHy 7 out of 8 ????  Well even though I fixed the ECU output, the Coilpack was too far gone and was only firing on 1 tower,  #6 and not #7. 

WHat I learned after all this.   Lotus or BOsch used some cheap IGBT on the board but They can all be replaced

THe ECU has 8 IGBT on the board but only 4 are being used (wasted spark)

THe ECU essentially has accomodations for 8 cylinder sequential spark but they didnt bother

After getting it running I hooked up the timing light again on EVERY wire "1 at a time" and could clearly see which cylinders are "firing" and which are dead. 

         I found that out of the 7 working cylinders, "Some" are strobing my timing light very fast, SOme are at 1/2 the speed, and #3 is misfiring !!

What does this mean?   I need to go in there and replace all 4 IGBT with stronger BIP373 or use the spares that are in there apparantly doing nothing, I also need new coilpacks

 

Where does this Story go Next?......................Megasquirt and LS1 coilpacks because it's about Damn time someone found an alternative to the Lotus ECU, but that will be in another dedicated thread. 

 

 

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img_25881.jpg

 

 heres the bad transistor being tested with a LED in place of a coil, it's not supposed to stay on cause that would be overDwell

 

img_25841.jpg

 

All fixed up

 

img_25861.jpg

 

img_25891.jpg

Edited by v8GTmac1
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On 28/09/2019 at 05:32, andyblackman777 said:

Michael,

Nice one!  Great work . Good you got it sorted without to much$$$$$

Mine has suffered some intermittent misfires etc. 

I wonder how the transistors failed? Faulty from new. 

All I can say is for those who have the infamous popping at idle to hook up a timing light to each wire 1 at a time and watch the consistency of the strobing light. 

I would love to know which part went bad first. 

If the igniter inside the ecu over dwells then it can and will damage the coils. 

But without an easy way to swap the coil packs left to right in order to diagnose, it leaves you with the easier solution of simply replacing both and hopefully that’s it

 

 

Edited by v8GTmac1

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Hi Michael,

This is very interesting indeed. I have been wondering about coil on plug options without too much knowledge just a view that they are probably better, how to achieve it is another issue. I did find some info on another user that had gone down that route but the controller seemed incompatible with the pencil coils chosen (it wasn't clear why just comments on the controller manufacturers website - whose name I have forgotten).

I played with water injection too on a previous V8 injecting into the intake pipework near your BOVs. Again there was a lack of science involved, all I can say is I was younger then and more enthusiastic than smart. 🙂

Due to all of that I am very interested in what you are doing and may look to copy you once you have it all figured out  and made it cheap and easy for the rest of us. 🤣

cheers

-Chris

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13 hours ago, cweeden said:

Hi Michael,

This is very interesting indeed. I have been wondering about coil on plug options without too much knowledge just a view that they are probably better, how to achieve it is another issue. I did find some info on another user that had gone down that route but the controller seemed incompatible with the pencil coils chosen (it wasn't clear why just comments on the controller manufacturers website - whose name I have forgotten).

I played with water injection too on a previous V8 injecting into the intake pipework near your BOVs. Again there was a lack of science involved, all I can say is I was younger then and more enthusiastic than smart. 🙂

Due to all of that I am very interested in what you are doing and may look to copy you once you have it all figured out  and made it cheap and easy for the rest of us. 🤣

cheers

-Chris

I think I read the same thread you did on the pencil coils. It was an AEM kit and was completely universal. 

Water injecting to me is just a security measure. You will never feel a gain from it but you know that it’s steam cleaning your pistons and bringing down detonation. 

Now if you want to add methonal to the same tank 50/50 and adjust your tune to it. Lower your fuel injection and increase your timing a bit. You will feel the kick when you turn up the boost. 

I have done things here in the simplest way I know how although it is more work then simply replacing what’s broken and fixing the oem ecu now that I know what’s failed  I still fear the secondary injectors may be leaking out  

only way to know for sure is to unplug them  

run rubber feed and return hoses to the rails  

prime the system and then pinch off hoses and monitor the pressure if it drops  

but again you need rubber hoses instead of the nylon to and from the main rail connections

 

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