[Mk2] [NA] Engine Hesitation Under WOT at High Revs - Rubbish MPG During Normal Use

[Loque]

Hi all,

I'm having a problem with my MR-2; it basically feels like it's hesitating under wide open throttle high up in the rev range (5k+) - It almost feels like you come off the throttle and back on very very quickly. I'm also averaging about 15MPG, even though I'm staying below 3k RPM.

It's running quite rough - the throttle feels like it's an on off switch when you're trying to maintain speed, as a by product it's lacking a bit in response too. The car also feels down on power. At idle the car feels smooth - I've looked at the engine and it's not vibrating in an untoward manner at all and it sounds as it should.

So Far I have:

• 
• Changed the HT Leads
  
• Fitted a new distributor cap & rotor arm
  
• Changed the Coil & Igniter
  
• Checked the HT Leads to ensure a good spark is getting through
  
• Read the fault codes [Nothing Codes were stored]
  
• ECU Reset
  

Yesterday I fitted a new distributor cap & rotor arm, checked all the leads over and reset the ECU. I fired the car up and went for a 10-15 minute drive. During this time, the car was fine although I kept the revs down below 3k as you generally do in normal driving. But the response was back and the engine felt much smoother. However, when the car was warmed up I engaged Wide Open Throttle in second gear and the hesitation cut back in at 6k. For the rest of the drive the car was back to how it was before. I've been unable to get the car back to this stage since - despite another ecu reset today it's still hesitant and pants on fuel.

I have read on here that it may be the oxygen sensor and that I should remove it to see how it reacts. How should it react if this is the fault, and precisely where is the oxygen sensor?

Does anyone have any ideas what the fault could be and how to rectify it?

Thanks for any assistance

[Loque]

I've done a bit more work on the car today in an attempt to ascertain the cause of the fault.

This morning I removed the spark plugs and checked them over. Three of them looked exactly like the below plug on the left. The one in cylinder two was the one on the right. Doesn't look like much I know, but maybe someone on here has some ideas:



I replaced the plugs in a different order to see if there was any credence to the idea that something funny is going on in cylinder 2. After this I took the car for a drive - unfortunately the car behaved exactly as before. A little bit lumpy.

I parked up and disconnected the lambda sensor in the exhaust with the hope that it would provoke some kind of reaction from the car. It sounded similar at idle, and on a drive around there seemed to be very little difference. I had to push a little harder on the accelerator to get similar performance, so I presume it's a little down on power.

Next up I'm going to bang some injector cleaner in the fuel in the hope that might clear something up. I've ordered a new fuel filter as well. I'm looking at running some wires into the car off the lambda sensor to check what voltages it's putting out. Maybe something will make itself apparent.

As always, any further ideas are always appreciated.

[4ndee]

It's a sensor, if re setting the ecu makes it fibe till its warm. Try ubplugging the lambda sensor and go for a drive, I had that problem on my n.a. it was overfuelling like mad.

[Loque]

As an update on this, the problem appears to have been sorted. As suggested I unplugged the lambda sensor and took the car for a drive; the car was not too differently at first, but eventually I began to notice that the throttle appeared quite unresponsive and the exhaust smelled of unburned fuel.

It should be noted that I got no 'check engine' light, but I did read the 2.1 fault code - this relates to the Oxygen Sensor, as you would expect.

After about thirty miles I decided to re-connect the sensor and re-set the ECU. Unfortunately, even after fifty miles or so the hesitation still appeared to be present. However the MPG improved dramatically to something close to about 30MPG.

As a last ditch attempt I used some BG44K. This has got rid of the hesitation - I can only presume that something was blocked in the fuel system somewhere, which the cleaner sorted out. I would recommend the stuff as it has fixed the problem, but some people have had bad experiences with it.

[4ndee]

You will need a new sensor buddy, the sensor is getting hot and going out of range, I recomend only o.e bosch sensors. When you unplugged it it runs on parameters which means instead of constantly switching it sticks at 1 voltage.

[Loque]

Yeah, I'm not up for wiring some shonky aftermarket universal one in there.

I'm hoping it will last long enough until I can afford a stainless steel exhaust, then I'll whip the whole lot out and lob it in the bin...

[bobhatton]

The lambda sensor only works on idle and very small throttle openings, after that the ECU goes open loop. So the lambda sensor is not the problem

[Loque]

Even now, sometimes the throttle works a bit like an on off switch when trying to maintain speed... Maybe that's the o2 sensor.

Still, the hesitation is gone & the power is back. I'm thinking of changing the fuel filter as a precautionary measure still.

[dan4]

The lambda sensor only works on idle and very small throttle openings, after that the ECU goes open loop. So the lambda sensor is not the problem

This is not entirely true...


Logue did you fix this? Im having a slight issue with hesitation from warmish starts and im sure its holding back slightly. Im starting to run out of ideas

[bobhatton]

The lambda sensor only works on idle and very small throttle openings, after that the ECU goes open loop. So the lambda sensor is not the problem

This is not entirely true...

Its not, please tell me more then

[Turbonoz]

The lambda sensor only works on idle and very small throttle openings, after that the ECU goes open loop. So the lambda sensor is not the problem

This is not entirely true...


Logue did you fix this? Im having a slight issue with hesitation from warmish starts and im sure its holding back slightly. Im starting to run out of ideas

It's true enough in this case. Under WOT at 5k RPM, the ECU will be in open loop.

As a general rule, the O2 sensor will most likely only be in operation up to around 4.5-5k RPM and halfway up the load scale. Some OEM ECUs run closed loop 100% of the time apart from WOT.

[dan4]

Whilst its true in part that the lambda probe isnt actively doing anything in open loop areas of the map it will still have a bearing on a fuel trim or adaptation across the map. It depends how far out the car is in closed loop really as to whether its noticed but I had a few years of dealing with this on Supra's. Its especially noticeable if you were to try it out with a piggyback ecu with a poor map, the trims are far more obvious

Now my probe is working ok when its scoped at idle, im going to struggle to scope it on the road due to the size of the scope rig. However I do have a simulator that im going to hook up which i can use to monitor. Im hoping that what ever is causing my small intermittant problem can be forced to become permanent with a bit of time

I notice that the na mr2 has only one trim whereas supras have two, is this the same for turbos as well?

[bobhatton]

There is a switch in the TPS that makes and breaks and puts the ECU into closed loop at very small throttle openings and that is all it does. There is no trim across any maps that I have seen.
Where have you seen this trim on the NA ECU, have you broken the ECU code?

[dan4]

Ive seen it on mk4 supra ecu's of the same year range and this will be no different albeit i expect a little less complicated as there only seems to be one trim applied rather than two. Although being a map system on the mr2 that may also have a bearing on trims

I have experimented with deliberate poor maps during closed loop cycles with a known good WOT afr, as the ecu learns the afrs swing out of where they were set in open loop as the trim is applied to the whole map. Its the whole idea behind it, to account for surroundings or wear changes to the engines air flow characteristics

I found this site useful when I was mapping and having trouble with these fuel trim gremlins, i had some more resources and a discussion I had with a specialist in the states but I cant find it at present

http://alflash.com.ua/vf1.htm

Now the problem here isnt mapping but suspected poor lambda readings but its kind of the same issues as ive experienced when mapping. Il add the car has a non genuine lambda probe which is my main reason initially for suspecting it of causing the problems

[bobhatton]

From what I read on that information it is saying “ECM is continually fine tuning the injector pulse width to keep the air-fuel ratio centered at about 14.65:1” . This is for idle only.
For power the AFR would be 11.0 or so and with the turbo under max boost it is 10.0 with the injector’s full open.

I would not be looking at the lambda sensor for any problem off idle when on open loop

[dan4]

It will perform slight adjustments in closed loop operations but will also store the adjustments as trims to apply as a new base value which will apply to a complete rev range/load range or you would find a cliff edge on the map. The adjustment for surrounding conditions/wear and tear isnt only required for cruising, its just not live adjustment when on open loop. Theres no point having a large percentage of adjustment only to have to do it all over again the next time the customer drives it. Hence ecu reset will change running characteristics, they learn fairly slowly ive found. I think i was doing circa 20-30miles to fully notice it

Ive now ruled out my lambda probe as it happens after using the simulator, went to the car after a short journey not fully warming up and when i went back to it i couldnt even pull away feeling far more like fuel starvation or the like with a horrid strangled induction noise. Becoming a little baffled by it

[Loque]

I wired up the o2 sensor and performed a test on it with a multimeter to see if it was responding correctly; I forget where I got the factory information from as it was a while ago (yet the BGB sounds like a good bet). Either way it
transpired that the sensor was, in fact shot. I obtained a genuine replacement and got it fitted.

This made the car run a lot better at high RPM and improved MPG, yet the low throttle 'jerkiness' was still there.

I've kept an eye on it, yet of late it has become very prevalent. An ECU reset used to correct the problem - for a few hundred miles at least - yet this doesn't really seem to do a great deal for any length of time anymore. It appears that the idle is a bit hinky as well. I know it's winter, but frequently it will idle at 2000-2200 rpm when cold - obtrusive to the point that when I push the clutch down the revs may rise to that level.

Hence of late I've been doing quite a lot of research on what could be causing it. The latest squeeze that I'm thinking is that it could possibly be TPS. I'd consider buying a new one, but I'm not sure if they come calibrated and to top it off some chump of a previous owner has rounded one of the screws that hold it in place.

[dan4]

Im as stuck as you are at the moment Logue, its an odd problem. Does yours struggle with warm starts too, mine starts but idles very low and then has no power for a few seconds. My cold starts are identical, they are improved with an ecu reset however

I can simulate tps so i may try that next although the readings im getting are spot on from all sensors

[Loque]

I can't really say too much about warm starts as I rarely do them. A while back it did have a massive problem actually turning over though which wasn't great. It went away when I began using 98/99RON fuel though and it's not too bad at the moment.

The two things that are on my mind at the moment - Throttle position sensor and Idle Speed controller. I may well be clutching at straws, but I can't actually think of all that much else that could be the problem.

[dan4]

No me neither. Try this if you can.....drive it until temp gauge gets to a quarter, shut it off and leave it for a few minutes then start it. See if you lose the throttle and get a low idle which all clears after a few seconds. If you do then we are on the same problem which I also think to be tps or the throttle body itself