Engine holding back at wide open throttle

[The Arch Bishop]

Hi all,

Wondering if anyone can offer advice on a strange issue I'm having. My old banger runs very well and pulls fantastically for its age and mileage, but if you go from about 75% throttle to full throttle, you can feel the engine holding back. The car idles and runs perfectly the rest of the time, it's just at full chat that it seems to struggle.

My first thought is the throttle position sensor, but I'm not sure on the procedure for testing it if that is even the issue.

Could someone with far more knowledge than me offer any suggestions?

[jonbill]

Is it slower at 100% than at 75%? Or just not 25% more?
Any popping and banging or does it just go soft?
Do you have Torque pro? Does the MAF values look reasonable going from 75% to 100%?

[The Arch Bishop]

Is it slower at 100% than at 75%? Or just not 25% more?
Any popping and banging or does it just go soft?
Do you have Torque pro? Does the MAF values look reasonable going from 75% to 100%?

Yes, that's a better way of putting it - It is slower at 100% than at 75-80%. It just goes soft at full throttle.

The only other symptom is if you change gear quickly from say 1st to 2nd - if you get back on the throttle too quickly, it kangaroo's briefly like the throttle can't keep up. Other than that it is fine and runs perfectly.

I do have Torque Pro but I'm not sure what the test should look like (I've never had to delve too deep into the app)- presumably a smooth-ish graph from partial to full throttle?

Any help truly appreciated!

[jonbill]

It does sound like it might be overfueling.
I would  expect as you say a smooth progression on the MAF as you go from 75% on up. Might have to get someone to test a normal one.
A couple of other things to maybe look at:
1. Ignition timing over the range. I wouldn't expect it to change much - should be low 30s I think.
2. Whether it is switching from closed loop to open loop when it degrades.
3. What the fuel trims are saying before it goes to open loop.

All these things are done by adding gauges in  'realtime information'

[m1tch]

Do you have an ODB reader to check the fuel trims? Might be worth doing an ECU reset - could also be a dirty MAF.

[shnazzle]

Sounds like vvti to me. Or maf
Cleaning the MAF is pretty much always the first port of call anyway

[The Arch Bishop]

Thanks for all of the advice so far everyone!

The MAF has been cleaned (although I guess that's no guarantee to it working properly) and you can clearly feel the VVTi smoothly switch over - it's still decently rapid up until you floor it completely then it just drops off slightly. It still accelerates, just at a diminished rate.

The battery was disconnected over the winter so the ECU will have reset, but it's still the same. I have checked for any historic codes when I first got the car - nothing in there and no reoccurring EMLs.

The car never smells fuelly either - to be honest, other than this slight issue, it really does run very well indeed.

It does sound like it's time to find my ODB and grab some readings. OK to post them on here? It's not my particular area of expertise (because I don't have any areas to be honest) so I may need some guidance about what I'm seeing from clever people! 

[Call the midlife!]

Could the cat be blocked/choked or an obstruction in the airbox etc?
Just throwing them out there, I'm far from expert but it sounds like it's struggling to breathe wide open?


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[jonbill]

Yes interested to see what you get out of the OBD.
Another quick check: it is throttle opening that's affecting things, not revs right? Peak power I bet is around 5500 so if you're pushing it much beyond there it does go downhill.

[shnazzle]

Could the cat be blocked/choked or an obstruction in the airbox etc?
Just throwing them out there, I'm far from expert but it sounds like it's struggling to breathe wide open?


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That was my next guess as well, choked.

Mine has a similar effect since removal of pre-cats to be honest. My theory being that at full load the precat chambers cause a bottleneck on flow and creates back pressure, resulting in a noticeable flattening of power at high load. Further visible by an identically setup car with an aftermarket manifold pulling away at the top end.
So, same theory holds for blocked cat or congested air filter. A clean air filter really can be night/day.
Especially the bumpiness you mention sounds a lot like it can't get enough air in at closed throttle. The IACV is a tiny hole and if the filter is restricted you're starving the car.

Fuel smell doesn't happen too often in a stock mr2 as the ecu is absolutely magical at the way and speed it adjusts fueling.

[The Arch Bishop]

Yes interested to see what you get out of the OBD.
Another quick check: it is throttle opening that's affecting things, not revs right? Peak power I bet is around 5500 so if you're pushing it much beyond there it does go downhill.

Yep, it's the throttle opening that affects it - I've not really gotten around to ringing the engine out - it's quite a torquey lump so I've not yet had the need! When I get a chance to get out for some tests, I'll record some data and see what is what!

Could the cat be blocked/choked or an obstruction in the airbox etc?
Just throwing them out there, I'm far from expert but it sounds like it's struggling to breathe wide open?


Sent from my iPhone using Tapatalk

That was my next guess as well, choked.

Mine has a similar effect since removal of pre-cats to be honest. My theory being that at full load the precat chambers cause a bottleneck on flow and creates back pressure, resulting in a noticeable flattening of power at high load. Further visible by an identically setup car with an aftermarket manifold pulling away at the top end.
So, same theory holds for blocked cat or congested air filter. A clean air filter really can be night/day.
Especially the bumpiness you mention sounds a lot like it can't get enough air in at closed throttle. The IACV is a tiny hole and if the filter is restricted you're starving the car.

Fuel smell doesn't happen too often in a stock mr2 as the ecu is absolutely magical at the way and speed it adjusts fueling.

From when I changed the manifold over, the general state of the exhaust (ports, precats etc) seemed really very good, so I'd be surprised if it was the main cat, but the air-filter is a possibility - I ordered a Blitz panel filter last night as it happens, so I will be able to eliminate/prove that soon enough! It's had quite a good service life up until the owner before me who did very few miles so didn't bother servicing it. I did give the current filter a cursory inspection and it was just about OK, but then they can still be badly blocked with contaminants.

Once again, thanks for all of the suggestions - some knowledge is a lot better than just flinging new parts at a car and hoping it sorts it. It's certainly less expensive!

[The Arch Bishop]

Found my OBD and had a quick play around on the drive. It seems I can rule my suspicion of the throttle position sensor out - it reads smoothly across the full length of the pedal travel up to 78% which I understand is correct.

There is a MAF reading in there as well - just the one so I'm guessing I'll need to be looking at that as well?

I will say that on the other settings, I'm not sure which details I should be measuring. Fuel trim long term bank 2 hurgle schmurgle....I'm a bit lost...

[jonbill]

Yeah that MAF one.
There are short term and long term trims for bank 1 and 2. Bank 1 is the cylinders that go with the right o2 sensor.
If everything is perfect, your long term trims will be zero and your short term trims will oscillate between -5 and +5 or so, while the ECU enriches and leans it out a little to check the reaction from the o2 sensor and make sure everything is working.
In practice, your long term trims may be non zero as the engine has learned to deal a little air leak or maybe aging o2 sensors.
So thats all good when your ecu is in closed loop mode which means it's constantly adjusting the trims to     Keep the AFR stoichiometric.
When you give it the beans it can't keep up with that so it just uses a fixed fueling map based on MAF, TPS and temp values (poss more, I'm sure the others will chip in). Thats open loop mode.

Open or closed loop mode is another thing Torque can read and show you.
I imagine you're going to be open loop when you go full throttle, that's why MAF etc values are worth looking at there
(Also on that note, might be worth checking sensible values for coolant temp: it'll add fuel if it thinks the engine is cold)

[The Arch Bishop]

Ah right, so if I put together a screen with the fuel trims, MAF, throttle position and closed/open loop, that should give me the info required?

Sorry for being dim - I've only ever used it for logging and resetting fault codes.

[jonbill]

Yeah thats the sort of thing.

[The Arch Bishop]

Cheers jonbill - I'll be changing the plugs and airfilter at the weekend after which I'll give it a go and see what comes about. Thanks for all the help!  :notworthy:

[Ardent]

T A B

Trims and MAFs are like when you have a check up, Heart rate, blood pressure, oxy saturation.
A very good window into whats going on inside.

Very good vid here plus part 2
https://www.youtube.com/results?search_query=secret+of+engine+problem+diagnosis-+fuel+trims+pt.1

[shnazzle]

78% seems a bit low to me tbh.
I could be wrong but I think 88% was my max.

I'll have a look tomorrow. Also, here is my torque layout. Might help. It's designed for landscape
You can import it

https://drive.google.com/file/d/1MwlIhWyDYqNHagMHGZm4iwdOiJnUusVt/view?usp=drivesdk

[The Arch Bishop]

T A B

Trims and MAFs are like when you have a check up, Heart rate, blood pressure, oxy saturation.
A very good window into whats going on inside.

Very good vid here plus part 2
https://www.youtube.com/results?search_query=secret+of+engine+problem+diagnosis-+fuel+trims+pt.1

I have to admit to thinking that vid would be well over my head, but it's actually very good at explaining it all. I'll pick up on part 2 when it's not gone midnight!

78% seems a bit low to me tbh.
I could be wrong but I think 88% was my max.

I'll have a look tomorrow. Also, here is my torque layout. Might help. It's designed for landscape
You can import it

https://drive.google.com/file/d/1MwlIhWyDYqNHagMHGZm4iwdOiJnUusVt/view?usp=drivesdk

Cheers for the layout - I've grabbed and imported.

Torque really was worth the pittance I paid for it all those years ago. It's saved me so much money over the years. One of the best tools you can get.

The TPS is at least linear over the length of the throttle, so I think it's OK, but your reading would be a great comparison.

Every day is a school day!

[shnazzle]

https://youtu.be/hpA2jWCfVjc

Ignore my fueling. My maf calibration is off. The joys of an aftermarket intake.

TPS goes from 11 to 84

[The Arch Bishop]

Cheers for that! Mine is also 11 at rest but peaks at 78 - but I suspect that the fact that it's linear is probably key. I was looking for it to rise steadily until around 80% throttle travel and then start backing off beyond that due to a fault, but it increases across the length of the pedal. The six % difference I suppose could be down to adjustment of the throttle cable which I will check.

[The Arch Bishop]

Curiouser and curiouser....

I have yet to log the various stats of my car, but yesterday, I replaced the spark plugs and the air filter then clapped my hands for a job well done and got on the beers at a BBQ. Today, I took it out for a test drive and it's not a happy banger!

At a guess, I'd say it's now down by about 30bhp and a load of torque throughout the rev range. It's as flat as a pancake like the previous wide open throttle issue but at all throttle openings. Along with this, going from part to full throttle quickly results and a large misfire and a lot of stuttering. I seem to have angered the MR2 Gods!

Going through things disturbed - coils (look like the original ones) plugs (obviously - these are the correct ones for the car), air intake (a lot of wiggling to get the air box lid off) and the battery (removed to change the gearbox oil) so a ECU reset in effect.

My initial thoughts are a coil being on the way out or the intake hose has a split that I can't see.

I realise that identifying and fixing faults on the internet is like nailing ideas to a Frisbee and throwing them over a rainbow, but any ideas on what to check next or what a failing coil would do would be great!

Annoyingly, no fault codes showing!

[jonbill]

I'd look at the o2 sensor readings and the fuel trims. That behaviour sounds like it's lean.

[Smcknighty]

Have you tried putting the plugs back? Your sure they are the right plugs?


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[Carolyn]

Sounds like you may have created an air leak on the input side while messing with the airbox.  Could be loose at the throttle body?
Also, having wobbled that lot around a bunch the MAF may have got dirty, or (if it was a bit dodgy already), the activity might have terminated its usefulness.

Maf issues often don't exhibit codes or warning lights.