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Discussion Starter #1
Hi, I am pretty much out of ideas for troubleshooting this problem perhaps someone may have experience of this.

I have a Tigra converted with a 2litre 16V Red Top engine(20XE). The engine is from a 1990 H Reg Astra GTE. The engine is unmodified apart from a lightened flywheel, Courtenay power cap (replaces the SFI box), stainless custom exhaust and a Pipercross Cone airfilter.

The engine conversion has been complete for a couple of months now, initially the head gasket failed, so I rebuilt the engine, replacing the gasket. All was well however the car seemed to lose the edge off its original performance.

I put the car on an Crypton emissions tester and found the values of CO, Hydro carbons and oxygen were way off. The CO was at 0.2% the Hydrocarbon PPM was at 1200 and the Oxygen reading at 8% at idle
I suspected an air leak and after invetigation found one of the ports on the inlet was letting in air. I replaced the inlet manifold gasket and ensure it was sealed using GM OEM gasket sealant. This stopped the leak.

Fixing the air leak improved the emissions partly, in that the oxygen emitted dropped to 3% and the Hydrocarbon PPM dropped to 300 which I would accept as fairly normal. However the CO dropped even further to 0.1%. The car has no CAT and adjusting on the CO screw on the air mass meter has no effect (I can make it go down but not up).

I am pretty sure there are no more leaks, and I don't understand why, if it were the leak at fault, why fixing it would make the values go down instead of up. I have changed the air mass meter, to no effect. I ran some redex through the fuel incase it was a clogged injector but also no effect

The car now 'kangeroo's' quite badly until reaching about 3500 rpm, when it seems to smooth out and drive normally, deceleration tends to bring quite violent popping from the exhaust. This symptom seems to be getting worse, to the point where the car is nearly undrivable

Speaking to Courtenay, they recommended changing the ECU temperature sensor, in case a false value was throwing off the ECU, however this had no effect when changed.

I have had this engine for about 4 years, it has been faultlessly reliable until now.
I am pretty much out of ideas, can anyone help?
 

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Discussion Starter #3
Yes, I have double checked the timing and it all seems to line up with the marks fine.
Is it possible a faulty crank shaft sensor could be responisble? The popping on the exhaust seems to suggest it is running too advanced.
 

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hydrocarbons are basically what petrol is. the fact they were high means the fuel wasnt being burnt, or there wasnt enough fuel for the amount of oxygen. high hydrocarbons could indicate overfuelling, but the very low CO content rules that out. high hydrocarbons can just as easily be due to running lean, you get to a point where combustion is so inneficient that a load of unburnt fuel and oxygen exits down the exhaust, this would explain the high oxygen, and would also explain why fixing the air leak reduced ppm, you took away its extra unmetred source of air. more severe mechanical probs like too much cam overlap, poor compression etc would also cause high hydrocarbons. a compression test and checking the cam timing more accurately wouldnt hurt (disregarding the crank pulley and pointer timing mark as its unreliable)

so most of the fingers point at underfuelling, you could address that by getting the fuel pressure measured (farly easy as there is a valve to do this on the fuel rail) since the fuel pressure regulator is basically just a spring and diaphragm, its possible for it to soften and supply less pressure. on from there get your injectors cleaned.

gixerboy makes a good suggestion too, an air leak would cause the popping and throw off emmisions readings.
 

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Discussion Starter #6
I checked over the exhaust throughly and found a small break in one of the welds, this is now fixed, but made no difference on the emissions or poor performance, but thanks for the idea.

Thinking about the problem I am less confident of the timing, so as my next step I will have this rechecked by someone better qualified than me.
If this is all ok then get the fuel pressure checked and have a compression test.

I would assume if it were a compression issue it would manifest itself in other symptoms such as pressurised or loss of water, oil in the water or pressurised oil system? Since the head gasket failed I have been watching this like a hawk and it seems to be holding its levels quite well.

Thanks for all your suggestions guys
 

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Discussion Starter #7
Thanks for your suggestions. The CAM timing has been double double checked. In the end we found that one of the HT leads was broken down and testing for additional vacuum leaks yielded some problems with the inlet sitting flush with the block. The inlet is now being sent away to be remachined.
 

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Jack O'Neill said:
...deceleration tends to bring quite violent popping from the exhaust...
my car has popping when you decelerate... had a powerflow system from the manifold... could this be a air leak on the exhaust?

also how do you get on at MOT time with not having a CAT fitted... i know the 20xe runs without one, but are they easily convinced, and do they run your car @ 1990 emission rules?
 

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Discussion Starter #9
Checked over the exhaust and did find a small gap in the weld which has been repaired. Usually get popping (on healthy cars) from running to rich/advanced on the timing.

According to my friend who is an MOT tester, he has to test the emissions according the vehicle the engine has come out of, so it was tested as a 1990 H Reg Astra GTE. I'm not sure how common it is accross the board but there may be some confusion as to what to do in the case of older engines in newer cars. If in doubt speak to your tester before the car goes in for MOT. Perhaps i'm a bit lucky having a tester as a friend :cool:

Rather annoyed at the prospect of having to have the inlet off again, but was speaking to someone who may sell me his throttle bodies fairly cheaply, has to be the way forward! lmao
 

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Discussion Starter #11
If its a pre-cat model you should be able to get it on an emissions tester and turn it down using the screw on the air mass meter. I think the CO value for the 20XE is 0.95% +/- 0.25%
Otherwise Oddball made a good point earlier in the thread about getting a pressure tester on the fuel rail incase of overfuelling (in my case underfuelling).
If it has a feline companion might be an oxygen sensor on the way out (although I am no expert with cat's).
 

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Discussion Starter #12
Found the problem. There was a small indent on the second inlet port, this was allowing air in. Had the face welded and machined (they found a 28 thou discrepancy, no idea how that happened). Initial vacuum checks didn't pick it up. Refitting tomorrow.
 

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Discussion Starter #13
Ok I was a touch over confident that would solve the problem. It hasn't. In fact its worse.
I have practically soaked the engine in brake cleaner trying to identify if there is another vacuum leak but I am confident we have now eliminated the air leaks.

I'm thinking fuel pressure/injector clogged/malfunctioning, faulty wire to the AFM or TPS or faulty TPS. Any thoughts?
 

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Discussion Starter #14
Right now I really do have the problem sorted. It was the air mas meter. Seems if you try to adjust the CO screw too high, it pushes the value beyond the expected range of the ECU, turn it down and eventually it gets back within range and you can adjust it normally. Very weird because normally I would expect an ECU light if the screw were wound in too far in.

Thanks everyone for your help and suggestions.
 
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