Exhaust popping, possible rich running

[Alex Pope]

Hi all,

I've had a 2002 MR2 for about three months and recently the exhaust has started popping/back firing on downshift.  It's not a huge amount but it's definitely not running right and it seems to be using fuel faster than I'd expect (but I've not carried out an MPG test).  The pre-cats were already gutted when I bought it.  The only error code I've had is the valve timing which was able to be cleared and is currently not present. It doesn't seem to be using oil or coolant.

So far I've tried the following:

- New NGK spark plugs
- Cleaned MAF sensor
- New air filter
- Checked O2 sensors for condition - they look reasonably new
- Checked O2 sensor feedback through OBD2 and Torque.  Can't remember the voltage but they both had very similar readings which suggests they are either both working or both not working in the same way (unlikely)
- Checked operation of coolant temperature sensor - it seems to be working perfectly
- The exhaust had started blowing at the outlet of the flexi joints (both of them).  I did a temporary patch fix today which seemed to reduce the exhaust popping slightly but no where near enough to suggest that the blowing exhaust was the main cause

I have read the Toyota repair manual which suggests checking the injectors, coils and fuel pressure (as well as the checks above).  I don't particularly want to buy new coils and injectors just in case and I don't have a pressure gauge to allow me to check the fuel pressure.

I'll check the fuel trim values at cold and 2500 rpm a bit later and post the voltages here in case this helps diagnose the problem.

So my questions are:
1) What else should I check and in what priority?
2) Is there a way to check the injectors, coils and fuel pressure?
3) What should the fuel trim values be?
4) What should be the O2 sensor voltages?
5) Any other ideas?

I'd be grateful for any help.  

Thanks.

[shnazzle]

Reset ecu? (battery neg cable off for 20 mins or so)
Sure exhaust is not blowing anywhere?
When you say both o2 sensors, do you mean bank 1 sensor 1 and bank 1 sensor 2? If so, if they're equal, either your cat is on the way out or your post-cat o2 sensor is not as good as you think.
There are 3 o2 sensors.

Trims for long term should be as close to 0 or between -5 and 5 as possible really. Short term can fluctuate more

[Alex Pope]

Hi shnazzle.  Thanks for the reply.

I did an ECU reset last night (took fuse out, waited about an hour, put fuse back in, went for short, varied drive).

By both O2 sensors I meant bank 1 sensor 1 and bank 2 sensor 1 (i.e. the two in the exhaust manifold).  I am of the understanding that  the third (bank 1 sensor 2) doesn't affect the running of the engine, rather it just monitors the cat performance.

My temporary exhaust fix removed the exhaust blowing noise and did seem to help but it is a patch of the end of the flexi pipes using exhaust tape and exhaust heat shield [edit: so it may not have worked as well as it seemed].  I have new exhaust section on the way from Cats4U - the piece that includes the main cat and two flexi pipes.  I did the temporary patch first to prove I'd found where it was blowing before spending the money on exhaust itself.

[Alex Pope]

Here are some readings. The exhaust was definitely blowing during this test.

IDLE

Fuel trim
Bank 1 Long term: 14%
Bank 1 Short term: -20%
Bank 1 sensor 1: -20%

Bank 2 Long term: 0%
Bank 2 Short term: -2%
Bank 2 sensor 1: -7%

Oxygen
Bank 1 sensor 1: 0.8v
Bank 2 Sensor 1: 0.75v / 0.09v (flipping between the two values)

Bank 1 sensor 2: 0.03v


2500 RPM

Fuel trim
Bank 1 Long term: -4%
Bank 1 Short term: -3%
Bank 1 sensor 1: -2%

Bank 2 Long term: 0%
Bank 2 Short term: -6%
Bank 2 sensor 1: -8%

Oxygen
Bank 1 sensor 1: 0.73v to 0.8v (varying)
Bank 2 sensor 1: 0.77v

Bank 1 sensor 2: 0.04v


RETURNED TO IDLE

Fuel trim
B1 LT: 14%
B1 ST: -20%

B2 LT: -5%
B2 ST: -1.5%

Oxygen
B1 S1: 0.09v then changed to 0.26v and stayed there
B2 S1: 0.09v then changed to 0.7v and stayed there

B1 S2: 0.73v

[Alex Pope]

So fuel trim long term/short term is significantly different on bank 1.

O2 sensor on bank 2 is showing odd readings.

The exhaust was blowing significantly during the test - definitely blowing from the flexi pipes, fairly certain not blowing from anywhere else.
There was a fair amount of light popping in exhaust; sounded a bit like misfiring but there's no hesitation/indication of misfiring at all when driving at any engine load or speed.

Other than fixing the blowing exhaust are these readings indicative of any other issue?

[Alex Pope]

Just been on 10 mile drive and everything now looks within limits - fuel trim remaining within +/- 5%, O2 sensors mirroring each other. Calculated about 37mpg so it looks like it's not running rich.

I'll change the exhaust and probably do the exhaust manifold at the same time then see how it behaves.

[jonbill]

Does it pop on the overrun too?

[shnazzle]

Hmm.. The fact that trims sort themselves out a bit while driving makes me still think that flexi ain't right on the right side.
Or the o2 sensor itself.

First test; swap the top two sensors. Reset ecu. try a full reset using the neg battery cable.  I tried the fuses and it didn't reset it properly

Check if the fuel trims have swapped banks after a decent drive

[Alex Pope]

The flexi is still definitely not right.  I just used exhaust wrap with a heat shield jubilee clip thing from Halfords.  Worked for 10 mins (exhaust note sounded great again - it's surprising how nice even the standard car sounds) then started blowing again.  It was a hopefully short term fix until I change the exhaust but was a lot more short term than I'd hoped!

It's popping mainly on the overrun; no pops, only blowing when accelerating or holding speed.  Pops most on overrun: low engine load and only just touching the throttle (say 30mph in 4th gear, 20 in 3rd slowing down for roundabout etc.), specifically when I then lift off fully and use engine for braking (i.e. overrun).

[Alex Pope]

To hijack my own thread I'm incredibly impressed with the handling.  Not pushed it yet as there's a slight wander and mismatched tyres but I can see that with this fixed it'll handle brilliantly.  Planning on changing out rear cross-member, lower suspension arms, possibly stiffer lowering springs, brake pads, brake lines at the same time as I do the exhaust.

I understand that the "thee amigos" on the exhaust can be a nightmare.  If I change both the manifold and lower section at the same time I assume this'll mean that I don't need to be concerned with not damaging these studs?  I assume the studs are either on the manifold or lower section of exhaust therefore the new manifold/lower section of exhaust will come with new studs?

[Ardent]

Reset ecu. try a full reset using the neg battery cable.  I tried the fuses and it didn't reset it properly

Could you elaborate on that please. What does the neg cable do over pulling the fuse.

Cheers
Jase

[shnazzle]

Reset ecu. try a full reset using the neg battery cable.  I tried the fuses and it didn't reset it properly

Could you elaborate on that please. What does the neg cable do over pulling the fuse.

Cheers
Jase

I haven't a clue! Doesn't make any sense.
I pulled both fuses to reset a code. Left it for ages. Did nothing.
So just stuck with what I know.

[jonbill]

Well... I think the popping and crackling may be a sign of worn valve stem seals and a little oil being sucked down under vacuum. Mine did it before I rebuilt the engine and doesn't do it now.
It must be possible to do valve stem seals without taking the engine out. Surely....

[Alex Pope]

Thanks jonbill.  If that were the issue would it cause any further damage by not addressing it?  

I'm not decided yet on which way to go with this car - I bought it to have something to play with but engine rebuild is likely a step too far as,although I'd love to take an engine apart, I don't want to spend £££s and I would think that to make a rebuild worthwhile it would need some money spent on it.  Not sure if I'll leave this car bog standard, turbo it or 2ZZ it, sell it after 6 months or keep it for 10 years!  If I decide to consider the turbo route I'd probably need to do a rebuild to ensure it's strong enough in the first place.  But that's all speculative.

I'm still hoping the main cause is the exhaust as the blowing and popping started at the same time - neither issue was present when I bought it.

[jonbill]

I think it would eventually kill the cat, if it is the case.

Seems a reasonable plan to fix the exhaust first and see if that does it.
After that, research changing the valve stem seals - if it can be done in situ it shouldn't cost so much.

[shnazzle]

If it makes you feel better mine pops a fair bit on overrun when the car is nice and hot.
Pretty much always does it on a down-shift when the exhaust is glowing

Never noticed how much it does it but I paid attention today and it does a fair bit of popping

[Carolyn]

Well... I think the popping and crackling may be a sign of worn valve stem seals and a little oil being sucked down under vacuum. Mine did it before I rebuilt the engine and doesn't do it now.
It must be possible to do valve stem seals without taking the engine out. Surely....

Not really... the cams have to come out to lift the buckets, leaving nothing to lever against to compress the spring.  Even if you could figure that one out, if seals have been leaking badly, the valve seats will need attention and the valve stems will need a big coke removal job, both of which require removal of the head.

It IS possible to remove the head with the engine in the car, but it's probably much easier to drop the engine and give it a good going-through??

The way you did it was (I think) the only way to guarantee a result.

[jonbill]

Ok, so aggregate advice might be:
1) sort the exhaust
2) if that doesn't fix it, live with it until it gets bad or you decide to go 2zz
3) rebuild the engine or replace.

[shnazzle]

What about the idea of switching the 02 sensors? Quick and cheap  s:) s:)

[jonbill]

Absolutely, stick that in before or after item 1!

[Alex Pope]

OK, thanks all.  Plan is as follows:

1) sort the exhaust (downpipe and cat arriving saturday)
2) if that doesn't fix it investigate O2 sensors further, start by swapping them over.  Check resistance of MAF, O2 sensors etc. to ensure they are still in range.
3) if that doesn't fix it, live with it until it gets bad or you decide to go 2zz
4) rebuild the engine or replace.

If I get any positive results I'll provide an update.

[jonbill]

What an excellent plan  :-) :-) :-)

[Alex Pope]

Hi all.  Quick update.  New cat section of exhaust fitted - this has cured the blowing and popping.  Thanks for all of the advice.

[jonbill]

Well done.