Exhaust "Chuffing"/Popping on Deceleration

[mmahon04]

Hey all,

After tracking down my hard warm starts and semi-rough idle (changing out my left bank injectors in the process), I went and changed out the right bank injectors.

A new problem developed immediately, however.

Symptoms
1) Re-checking AFR's after installation, they seem to be bouncing in quite a large range now. Whereas 14.2 +- .1 was normal before, now I see 14.1=-.4 (i.e. 14.5 - 13.7 fluctuating).
2) After the warmup process (idle starting higher and settling), I now hear an intermittant, irregular "chuffing" from inside my exhaust. Not a pop. Not a backfire. More like a low bass note...a "chuff" if you will. This noise continues regardless of having my O2 sensor connected or not. Idle drops slightly when the "chuff" noise happens. I might see if I can get an audible video of it, but I think everyone knows how difficult that might be.
3) On throttle liftoff, I oftentimes now get a small pop or couple of pops. Not very loud at all, but noticeable at lower speeds. Not all the time either, but definitely there now whereas it wasn't before.
4) Car seems very "boggy" off idle and down low.
5) Seems much easier to stall car when just fired up now. Pulling out of parking space I nearly stalled it and had to give it much more throttle; stopped the car and the idle bounced up above normal before oscillating a bit and settling down.
6) Seemingly no issues when on throttle and moving.

Notes from when I changed out right bank
1) AC compressor had to be partially removed to access front fuel line -> rail fitting connection.
2) Rear fuel rail -> fuel damper connection is easily accessible with airbox cover removed.
3) Cleaned plastic on electrical connectors to injectors with MAF cleaner (same as I did on left bank).

Things Checked So Far
1) Replaced right bank injectors back with original leaky injectors. No change in issue.
2) Had spark plugs replaced; they looked horrible, and was told one was not fully seated. Condition is unchanged.
3) Checked vacuum lines on back of throttle body (particular the 'Y' leading to the fuel pressure regulator and fuel pressure damper). No problems.
4) Fuel injectors all seem to be firing; I can feel their off-on 'pulsing'.

Questions:
1) Can any fuel that leaked from the lines trickle down the fan shroud and settle near the bottoms of the intake manifold, and compromise the gaskets there?
2) Likewise, can any MAF cleaner I used possibly disintegrate the intake manifold gaskets?
3) Can a fuel line/rail have an air pocket?

I'm going to check for vacuum leaks this weekend, and also double-check that my wideband standalone O2 roughly agrees with my O2 sensor readings (want to rule out tool error). I'm kind of at a loss though. I have absolutely no idea why this would crop up immediately upon changing the right bank injectors, and have the problem still remain despite going back to the original injectors for that side. Nothing else has been touched. Anyone else have any ideas or heard something similar? If Steve Wong or Steve Weiner is out there, a chime in from a professional would be highly appreciated at this point.

[Mr Project]

Have you checked the head studs already?

[Bob Kontak]

Originally Posted by mmahon04

I have absolutely no idea why this would crop up immediately upon changing the right bank injectors, and have the problem still remain despite going back to the original injectors for that side.

Did you touch any of the intake runners where they bolt into the heads on the right side? Are the nuts tight?

If you did not touch the exhaust, I am pretty sure you have an air leak near the cylinders.

Since you messed with the right side last, you know where to look.

[mmahon04]

Originally Posted by Mr Project

Have you checked the head studs already?

Valve adjustment quite recently, and PO had the studs replaced within his ownership (with past 10k miles or so).

[mmahon04]

Originally Posted by Bob Kontak

Did you touch any of the intake runners where they bolt into the heads on the right side? Are the nuts tight?

If you did not touch the exhaust, I am pretty sure you have an air leak near the cylinders.

Since you messed with the right side last, you know where to look.

We'll see tomorrow. The more I systematically look at my symptoms, I feel as though the car is running real rich.

[Bob Kontak]

Originally Posted by mmahon04

The more I systematically look at my symptoms, I feel as though the car is running real rich.

Fair enough.

Only time I had chuffage was when #4 intake runner was not secured tightly.

Pulled my hair out - long time.

Starting fluid was used as a cheater test to find the leak.

[T77911S]

you could have introduced an air leak on the right side. injector sleeves, Orings, intake botls.

the other thing is the problem could be on th eleft side. you may have fixed the right side and it is working better but now the left side could be the issue.
here is why, right side is fine but left could have an air leak making it lean, you richen up the other cyclinders to compensate for one or more lean ones. the over all mixture is just a compromise of all the cylinders mixture.

new plugs are a good oppertunity to check mixture.

if fuel runs down and can be sucked in at an intake gasket, you have a vacuum leak.

get some carb cleaner or starter fluid and check for leaks. water works too.

[mmahon04]

Well everyone, I spent some serious time with it today. Bought the full assortment of sprays/fluids to check for a vacuum leak: brake cleaner, MAF cleaner, throttle body cleaner, carb cleaner, and starting fluid. I think my problems were/are threefold, in order of likelihood:

1) Dirty throttle body/AFM.
2) Lazy O2 sensor.
3) Air leak.

Here's what I did:
1) With car running and hot, I disconnected the O2 sensor, and connected my multimeter leads to it and a ground. Highly variable readings; all within spec, but bouncing quite a bit.
2) I had read that carb cleaner is plenty volatile while also being less explosive than starter fluid and the other cleaners. Not knowing the exact effect carb cleaner would have in relation to a vacuum leak, I sprayed some on the air filter to see how it would behave when "ingested." Idle drops slightly, then bounces back up higher, then re-settles.
3) I removed the airbox cover and filter entirely and sprayed a spurt directly in. Idle dropped more (I made it stall a few times, intentionally), then bounced back up to re-settle again. At this point I noticed the "chuffing" was gone...
4) I sprayed all around the intake runners, the rubber boot to throttle body, around the ICV, around injectors, etc. Damn near soaked it all. No change in idle or otherwise.
5) Looked at the multimeter, and instead of bouncing all around, it was sitting pretty evenly now around ~.62 VDC.
6) Plugged in the wideband, and calibrated AFM A/F mix to 14.2 +_.2. Less variability there than before as well. Fully tightened on the AFM screw put me at ~13.6 AFR; I believe I'm about 1.25 turns or so backed out.
7) ICV is new, but shut off the car, removed the hoses to it, and turned ignition to position 1. Vibrates, and looked inside; all good, but cleaned it anyways. Also compared to my old ICV, with both behaving the same when plugged in.
8) Installed the right bank injectors. Made sure to jumper the fuel pump to pressurize the lines without cranking the engine. No changes in car running versus how it was running after actions 1-7.

Results:
1) Bog is gone.
2) Popping on coast is gone.
3) "Chuffing" is vastly reduced (~50%+ reduction; less frequent and less prominent).
4) AFR's more even.

Why I think the O2 sensor is lazy:
1) When I plug the O2 sensor back it, it immediately gets choppier. Not bad though; I think I've heard but one muted pop today (also at huge throttle transition blip, not just coasting), but the "chuffing" at idle is there. Again, still not nearly as bad as before though.
2) Disconnect O2 sensor: rock solid idle, no chuffing.

Why I think I could still have a minor air leak:
1) Increased variability in AFR's. With O2 sensor disconnected, used to be 14.2 +- .1, not 14.2 +-.2

Going forward:
1) Debating getting a new O2 sensor. Previous owner had new spark plugs installed, valve adjustment done, and new O2 sensor installed at shop local to his area. So far, I've had to re-do the valve adjustment (was not done right), have the spark plugs swapped (poor quality, not fully tightened), so it would not shock me at this point if the O2 sensor was truthfully never changed, or if it was, an off-brand was used. I might have partially carbon fouled it as well: when I installed the injectors (either bank, prior to today), I did not jumper the fuel pump to re-pressurize the lines prior to firing the engine up. All times I had done the injectors prior to today, this resulted in a big cloud of white smoke: the result of fuel vapor/fuel that had been pumped in and blown out without igniting in the engine.

Originally Posted by Bob Kontak

Fair enough.

Only time I had chuffage was when #4 intake runner was not secured tightly.

Pulled my hair out - long time.

Starting fluid was used as a cheater test to find the leak.

I think we might both be right Bob. I think the O2 sensor, being a bit slow, is leaning and richening on a 'delay'; would explain the previous bog (symptom of rich) while also explaining the popping on deceleration and throttle transitions (symptom of lean).

Originally Posted by T77911S

the other thing is the problem could be on th eleft side. you may have fixed the right side and it is working better but now the left side could be the issue.
here is why, right side is fine but left could have an air leak making it lean, you richen up the other cyclinders to compensate for one or more lean ones. the over all mixture is just a compromise of all the cylinders mixture.

Agreed. I still do have this concern, but considering I had zero issues after I had previously installed the left side (and AFR's were rock solid), I think it would be unlikely (although not impossible) to have one of those injectors have an issue immediately after changing out the right side, and still continue after changing back. I do have a concern that the location of the O2 sensor bung is going to be more heavily influenced by the output of the left bank rather than both; I think I might have an O2 bung welded in on the right to mirror the left; get a reading from both sides then. Might also allow me to see instantaneous AFR's while the car O2 is still connected and in the exhaust stream.

[porsche930dude]

i didnt read everything but did you check the decel valve . the main purpose of it is to smooth out the chuffing

[Bob Kontak]

Originally Posted by porsche930dude

i didnt read everything but did you check the decel valve . the main purpose of it is to smooth out the chuffing

The purpose of the decel valve is to introduce air past the throttle plate when you let off the throttle at high RPM so it can drop at a more controlled rate rather than instantly.

I don't think chuffing / puff puff puffing at idle is the reason it exists.

Also, forgive my ignorance, but does the 88 have a decel valve? Our SC's do but does the 88 with Motronic?

[mmahon04]

Originally Posted by Bob Kontak

The purpose of the decel valve is to introduce air past the throttle plate when you let off the throttle at high RPM so it can drop at a more controlled rate rather than instantly.

I don't think chuffing / puff puff puffing at idle is the reason it exists.

Also, forgive my ignorance, but does the 88 have a decel valve? Our SC's do but does the 88 with Motronic?

Negative. ICV and MFI system is designed to control idle fall without it.

[mmahon04]

Well, my problems are largely unresolved. I think they were more temperature related than anything else, truthfully.

Yesterday fired the car up (70oF afternoon) and went for a drive. I'm taking greater notice of an initial "bounce" in engine speed on fire up. Engine speed seems to oscillate for first 4-7 seconds, going from low (significantly less than 880) to high (~1500) RPM. Also, the popping and roughness is very pronounced at cold startup and for first 7-10 minutes of driving, progressively decreasing while the car is warming up.

Went to fire the car up today (49oF morning), turned over, caught high (~1500), then engine speed dropped to nothing; stalled out. Did it again; engine caught high, then fell to stall. Had to give it throttle to have it stay running. Within a few seconds of a "forced" fast idle (feeling rough while doing such), the car would stay idling, albeit as rough as I mention in earlier posts.

Starting to get at my wits end.

Anyone have any further thoughts? I plan on checking startup and running fuel pressure, swapping to a good O2 sensor, and checking AFM temperature sensor resistance, but beyond this I'm stuck. What could possibly be causing all these symptoms?

[T77911S]

oh, you have a 3.2......

just some things to think about:
when cold, it is in open loop mode
CPS can cause strange sysmptoms. never thought it would cause rogh running, might even argue it, until my 88 325is was fixed with a new CPS. runs better now
O2 only good for about 100k miles
wiring/ground connection could be intermittent
how do the plugs look
exhaust leak
fuel pressures/regulator

[Rich Gas]

Ok, I have a 3.2 and here's what I did:
- first I cleaned things afm, throttle body, ICV, added swepco 503...
- second I threw money at it, changed fuel filter, cap, rotor, plug wires, new injectors, coil, air filter, O2, CHT, Cat
- third I realized it runs rich during the cold winter, and leaner during hot summer
So, I took off the AFM, yanked the cap off, lean it out in the winter, richen in the summer, spring.
Works....

[scarceller]

Unplug the O2 and drive it, does the decel popping go away? If so get a new O2.

Exhaust popping on decel is actually the result of fuel still burning when exhaust valve opens. Decel has very high vacuum conditions and this causes the cyl to only slightly fill with air fuel. Under these conditions the Flame front speed is greatly reduced and mixture tends to burn slowly. The DME actually compensates for this by really advancing ignition to fire the plug early. But if the mixture is slightly leaner than usual it causes the condition to get worse and more noticeable. Also many non stock exhaust systems make this popping condition very noticeable and in some non stock setups you may not be able to totally eliminate it.

The fact that your engine did not do this before tells me that during decel your mixture has gone more lean than usual. The O2 sensor being lazy can certainly cause this so I suggest disconnecting it and see if popping goes away.

[scarceller]

Originally Posted by mmahon04

I'm taking greater notice of an initial "bounce" in engine speed on fire up. Engine speed seems to oscillate for first 4-7 seconds, going from low (significantly less than 880) to high (~1500) RPM. Also, the popping and roughness is very pronounced at cold startup and for first 7-10 minutes of driving, progressively decreasing while the car is warming up.

This is usually the result of lean base mixture, air leak or poor fuel pressure. Bottom line is that the motor is most likely lean. Also the CHT sensor can cause this.

Try this: using the WBO2 meter set base mixture at 13.8AFR and disconnect O2. Leave car overnight and then repeat cold start with O2 disconnected and sligthly rich base mixture. Did cold start improve? If so you have clear indication of a lean mixture.

[mmahon04]

Originally Posted by scarceller

This is usually the result of lean base mixture

With O2 sensor disconnected, WBO2 read 14.2+-.2; should be okay.

Originally Posted by scarceller

air leak

Sprayed carb cleaner and throttle body cleaner over nearly every single connection, hose, junction, etc. No change in idle, although I did have the ICV still connected when I did this. I read I may get better results with it disconnected though.

Originally Posted by scarceller

or poor fuel pressure.

Don't have a fuel pressure test kit yet; I think I will have my mechanic test this.

Originally Posted by scarceller

Bottom line is that the motor is most likely lean.

I'm not getting any kind of lean surge or bucking though, which is why I am so confused. It's as if I am both rich and lean at the time time. Rich for bogging and idle dipping, lean for popping and chuffing in exhaust. Base mixture was originally set lean when I purchased the car, and I could feel the bucking when moving on/off throttle. Sitting still in neutral, any throttle at all would shoot the RPM's to nearly 2k; I had very little modulation at that point. I had fixed that issue very early on.

Getting back to this problem though, all these symptoms get considerably better the warmer the car gets.

Originally Posted by scarceller

Also the CHT sensor can cause this.

CHT tested within spec with car hot last time I checked (after doing left bank injectors). I will check again both hot and cold though.

Originally Posted by scarceller

Try this: using the WBO2 meter set base mixture at 13.8AFR and disconnect O2. Leave car overnight and then repeat cold start with O2 disconnected and sligthly rich base mixture. Did cold start improve? If so you have clear indication of a lean mixture.

I'll see if I can do this this next weekend. I don't have the time/space to get that involved after work M-F.


I do appreciate your input.

[mmahon04]

Originally Posted by T77911S

CPS can cause strange sysmptoms. never thought it would cause rogh running, might even argue it, until my 88 325is was fixed with a new CPS. runs better now

I'm looking to investigate the speed sensor. If it's weak/failing, I can see how it would need to be closer to get a good signal to provide accurate engine timing. Considering heat makes things expand (thus bringing it closer and providing a better reading), I'm going to check it out.

Originally Posted by T77911S

O2 only good for about 100k miles

New one on the way. Was supposedly recently replaced, but will definitely know now.

Originally Posted by T77911S

wiring/ground connection could be intermittent

Will be cleaning intake manifold ground sometime today.

Originally Posted by T77911S

how do the plugs look

Old plugs (I will post pictures) were a) low quality, b) two were not fully seated, and c) right bank had some oil contamination. Will be having mechanic re-pull my new plugs to check them out with me though. That aside, the ceramic core looked perfectly light brownish on the left bank; right bank was little harder to tell with the deposits.

Originally Posted by T77911S

exhaust leak

Will be checking exhaust manifold sealings, but considering this popped up immediately upon changing right bank injectors, I'm hesitant to think this is an issue.

Originally Posted by T77911S

fuel pressures/regulator

Will be having mechanic check fuel pressures.

[mmahon04]

Well, I got a bit of time and checked out a few things tonight that I hadn't yet inspected:

Within an hour of car shutdown:
1) Air Temperature Sensor: Tested at 822 ohm (and rising). Spec is 2k-3k ohms @ 20oC, 250-400 ohm @ 80oC.
2) CHT Sensor: Tested at 90 ohm (and rising). Spec is between 90 and 6.8k ohms.
3) Crankshaft Speed Sensor: 1208 ohm, ±1. Spec is 600 to 1.6k ohms.
4) Reference Sensor: 1218 ohm ±1. Spec is 600 to 1.6k ohms.

Other thing checked:
1) Distributor cap and rotor. Considering I had replaced these within the past year, I hadn't looked here yet. Popped off the cap however to find 2 of the distributor points heavily scorched and pitted, with all the others with some considerable carbon buildup. I even tried to check resistance across the scorched/pitted point: I had to set my multimeter to the 20k ohm resistance range to even begin to get a reading! Needless to say, I've got a new cap and rotor ordered and should be here Thursday.
2) Spark plug wires. I'm not a fan of these thick Magnecores despite the okay reviews, and one on the left bank didn't seem to be fully seated.

I'll re-take all measurements tomorrow when the new O2 sensor comes in and I get that installed.

[T77911S]

as long as the gap for the CPS is spec, moving it closer has nothing to do with it.