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So i need get my feet wet...

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Old 08-23-2016, 09:36 PM
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So i need get my feet wet...

So this is where i need get my feet wet, lol.

I recently bought a RX8 with problems from neglect. Now i know that scares the most of you but i am a biker hand have dealt with more engines than i have had hot dinners and i feel this information needs be out their on the internet.

The rotary engine seems to be damaged particularly by lean conditions.
The list for these conditions is very long as this engine has very little parts internal to it but in fact they are all external and the internals are actually quite sensitive.

Any solenoid (thats a rotary solenoid, very different to ordinary solenoids for potato engines) are critical. The 3 valves which are controlled by vacuum for two of them and a motor for the third are critical for operation.

The front so called O2 sensor is NOT an O2 sensor but and AF sensor which is in fact two O2 sensors back to back measuring a wide band fuel mixture. The rear O2 sensor which most Diag tool read is in fact the one stuck in the middle of the cat.

Do not rely on these readings from this sensor it is a after cat deal.

you have 3 ports that affect fuel trim on this engine and the downstream O2 sensor wont help you. AFR readings are what you need seen as this engine uses wideband sensors. If you have a engine issue and you keep running the engine you will destroy your engine diagnosing it as many garages do and also as many MOT stations are guilty for destroying this engine. These procedures are wrong, very wrong.

The engine is a simple pump burn and out engine like a 2 stroke motor so two stroke oil in the tank will burn out the AF/O2 sensor which is not an O2 sensor i just said that so you know what i talking about.
The cat is one of the best CAT's out their but is not indestructible so be ware.

Getting a perfect running car through the MOT emissions is like trying to do surgery on an animal without anaesthetic. You drop it off walk away to come back to a very damaged car. Rotaries are not to be idled for long or revved at constant fixed revs(part of the MOT test).
Every MOT test damages your rotary car, simple.

As soon as you feel a rough idle have everything checked such as fuel pressure, vacuum leaks, exhaust leaks, backfires and popping(this kills apex seals but sounds cool for short time), fuel trims, sensors, vacuum solenoids and so on.

Fail to do this and the engine will suffer. I just had the MOT station screw my RX8. It has a limited slip diff and they run a standard rear break roller test, run the engine for over 40 min without a fan to cool the engine and on top of that flooded it 3 times then did a emission test which failed.
The MOT tester warped my battery cells by cranking the engine so much while trying to pump the accelerator pedal to start it, i had to tell him to stop and just try without padding the accelerator and it started.

Am I the only one who wants to kill the MOT tester?

Anyone who owns an RX8 must pay attention to all these issues immediately before it is too late. Stay and wait for your MOT never drop it off and leave and come back.

2 Stroke engines don't need a CAT so why should rotary engine?

They both have no valves, cam shafts and need cylinder pre-mix by pump or in tank...

You cant fix stupid. MOT testers are stupid.
Pinking damages these engines by the second so you need know how to detect it, marbles rattling in the housing is primary sign, check your exhaust and learn to drain old fuel.
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Old 08-24-2016, 07:10 AM
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Hey, welcome aboard.

The front 02 sensor is a wideband, like most cars this side of the year 2000. The rear is for emissions control/cat health, which I'd expect to be what a MOT test is interested in.

The engine does need a cat because of the NOx and unburnt fuel it puts out.

And your car should not flood if it's healthy. Especially when warm. If they had trouble starting it warm, you already had low compression before you got to the MOT station.
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Old 08-24-2016, 09:52 PM
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Originally Posted by Loki View Post
Hey, welcome aboard.

The front 02 sensor is a wideband, like most cars this side of the year 2000. The rear is for emissions control/cat health, which I'd expect to be what a MOT test is interested in.

The engine does need a cat because of the NOx and unburnt fuel it puts out.

And your car should not flood if it's healthy. Especially when warm. If they had trouble starting it warm, you already had low compression before you got to the MOT station.
I should have mentioned he tried pulling of the brake test rollers at very low revs. As i have found out 1500 rpm and dropping the clutch with this car sets you off like a rocket and seen as he nearly drove into his canteen area first time he is now adding very little gas.
I have gone from the front to the back of this car and yes emissions failed due to the back honeycombs being smashed and melted from a speed hump damage by previous owner. I had to replace front drop-links and was wondering why there was scraped damage on the exhaust tubes.

The rear O2 sensor is actually only testing the very first honeycomb of the CAT and not the rear section. Yes the broken parts of the rear CAT had passed down the tube and plugged the muffler. This has caused a rich but lean condition due to TPS reporting wrong against the air flow and fowling out plugs, overheating the intake air temperature. This then burnt out the AFR sensor.

Hopefully i have caught it in time but we shall have to wait and see. This flagged up NO check engine light, go figure. Still the class of TAX and MOT emissions test equivalenced to a 2.6 Litre engine is the wrong tests for this engine.

The test needs to be far more related to a 2 stroke engine if they are not going to create a special class for the Rotary. Just saying.



Thank you for the reply and welcoming me to the forum.
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Old 08-25-2016, 12:12 AM
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Originally Posted by Fireicer View Post
Yes the broken parts of the rear CAT had passed down the tube and plugged the muffler. This has caused a rich but lean condition due to TPS reporting wrong against the air flow and fowling out plugs, overheating the intake air temperature. This then burnt out the AFR sensor.
I'm not sure how you figure that works, but a plugged cat causes high EGT and subsequent engine damage. You really might want to have a compression test done. The cat damage need not be from a speed bump, it could be from less than healthy ignition coils, which is fairly common on these cars. The TPS, intake air and AFR sensor have nothing to do with this. Long story short, read up here about ignition failure symptoms and solutions. It's the leading cause of engine/cat problems.
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Old 08-25-2016, 09:06 AM
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Originally Posted by Loki View Post
I'm not sure how you figure that works, but a plugged cat causes high EGT and subsequent engine damage. You really might want to have a compression test done. The cat damage need not be from a speed bump, it could be from less than healthy ignition coils, which is fairly common on these cars. The TPS, intake air and AFR sensor have nothing to do with this. Long story short, read up here about ignition failure symptoms and solutions. It's the leading cause of engine/cat problems.
Yup, Ignition coils were faulty when i obtained the car. The exhaust had a leak just before the back box muffler and was backfiring allot due to that so i think that contributed to the CAT damage. The reason i mention all the other sensors is that with a plugged exhaust the gasses cant escape the engine which causes exhaust gases to be present after the rotor has passed the exhaust port and heading for the intake. A partial amount of exhaust gases are present when the rotor reaches the intake port making for less air and fuel mixture and changing the AFR reading. This is normal fault for all cars i would have thought as it is exactly what happens to a piston engine in the same condition. I agree engine damage may have been caused by this, previous owner neglected car. He kept it well polished and clean but not so much attention and love to the mechanical side. Engine rebuild at some point maybe needed, i got to redo tests and so on. Compression is currently within tolerances as the test has been done before it's MOT but the melted CAT and plugged muffler was not detected when that was done.

I had several mechanics tell me it is normal for a RX8 to backfire like that so instead of going under the car and smoke testing the exhaust i left it until after it's MOT to double check.

Last edited by Fireicer; 08-25-2016 at 09:10 AM. Reason: needed to add an addition for completeness
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Old 08-25-2016, 05:55 PM
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Originally Posted by Fireicer View Post
with a plugged exhaust the gasses cant escape the engine which causes exhaust gases to be present after the rotor has passed the exhaust port and heading for the intake. A partial amount of exhaust gases are present when the rotor reaches the intake port making for less air and fuel mixture
Fortunately this isn't a normal piston engine, or a two stroke engine, so what you're saying isn't what's happening. There is zero overlap on the MSP so exhaust gasses physically can't be pushed back into the intake chamber. Exhaust back-pressure will increase pumping losses, which causes an apparent power loss.
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Old 08-25-2016, 08:37 PM
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Originally Posted by Legot View Post
Fortunately this isn't a normal piston engine, or a two stroke engine, so what you're saying isn't what's happening. There is zero overlap on the MSP so exhaust gasses physically can't be pushed back into the intake chamber. Exhaust back-pressure will increase pumping losses, which causes an apparent power loss.
Well my issue is after all bases covered. No misfires or stumbling what so ever in normal running conditions, if you let it sit idle for too long as is done in an MOT station it begins to rumble and stumble yet when you drive it and give it a good flow of air around the car this goes away. Watching my fuel trims which sit normal around 0-3% on the long term and fluctuate due to driving conditions on the short term all is fine unless it idles for 15 20 min which the MOT station keeps doing to warm engine up for the emissions test. And yes no cooling fan or nothing just ticking over on the lifter. Car drives perfect throughout the rev range just is a mess at idle if stationary for too long. Just cant get the MOT guy(even when he knows about this issue) to get of his high horse and test it before it begins to overheat. Seems like he wants it to overheat into the 90-100 before he does the test which is totally unnecessary. Before i took it to the MOT compression was good but now it is delayed starting even from cold which never was before the test. UK MOT centres brake peoples cars. It has happened with other cars i have had too. Glazed cylinders, slipping clutches and so on only after the test. They have no respect for someone else's car and treat it like garbage. If compression was screwed during the MOT test then why does it run perfectly through the rev range? Surely a misfire or bad fuel trims would be shown at revs above 850rpm. It's almost guaranteed my plugs are fowled up after this MOT test for sure. I need cover a good 50 miles to wash them out in fairness but can't do that till test is passed.

Last edited by Fireicer; 08-25-2016 at 08:40 PM.
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