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Exhaust temperature, i need some tips.

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Hello everyone.

Im busy setting up the standalone in my GTO (Mitsubishi 3000GT) and i noticed the burger light come on (cat temperature warning light).

even when i cranked the AFR down to 13.0 it was still cooking. i moved the AFR back to 14.0AFR or 0.97 lambda its okay at idle but when i try drive it it immediately lights up warning light. also car wont rev past 1500 so something is up.

so i jumped on 3si.org (the 3000gt forum) believing the ignition timing was out of spec. i found there was a very handy table for recommended ignition timing. since my engine is stock i figured that should be fine.

so the next thing is valve timing but that seemed where it should be.

i did do the calibration of the LC2 wideband so i dont think its that.

so my question is this. if the AFR is correct then the exhaust temps should be normal? the forums recommend a 15.2 AFR for best fuel economy but on a 90s V6 i dont think fuel economy best fits this build. my aim is between 13.7 and 14.0AFR as i think its a safe range.

the goal here is "just make it drive" not chasing power yet so reliability is my concern.

if all the above is fine then all that is left is to conclude the heat warning on the cat is faulty, not an assumption that one wants to get wrong and i dont think the sensor is faulty. before the old factory ECU blew up it would come on now and then when i floored it hard for more than say 30 seconds. but now its on all the time.

how can i check that my exhaust temps arent too high and wont break the car?

i figured its best to ask for help before i blow something up.

Edit: to clarify i dont know how hot it is getting just that the catalytic converter over temperature warning light switches on

It may be partially blocked, or contaminated and trying to burn that off.

A common problem that leads to an over-heating cat is an ignition misfire, as the unburned mixture is 'burned' by the cat'.

If the cat's the original part, it may be well past replacement with a good performance oriented cat' - especially as overheating (if true) can warp the cat'.

okay cool so check for misfires and then and blocked cat.. right ill look into that. i was afraid there was something wrong with the tune. cause im sure i followed the steps outlined properly so i was quite surprised when the heat got so high.

perhaps adding a proper EGT sensor might be best. car only had 53 000km on the ODO so i want to keep it a long time. and maybe a cat replacement is a good idea.

when i get a gap ill have a look at the other things and report back. but what you say make sense i didnt think the cat could be the issue itself. my first thought was its something i did but that doesnt seem to be the case here. always better to confirm rather than assume.

thanks for the input greatly appreciated

it is a common issue for these cars as Gord mentioned above the cats get partially blocked but also the sensors do wear out and are known to come on early when this happens, checking the cat would be the first step

i would be setting the Lean Cruise afr somewhere in 14.5 to 14.7 on this engine with 88-95 octane fuel and 14.7-15.1 with better fuel

Regards Ross

thanks for the help.

im going through the HPA practical tuning now looking at the Syvecs worked example. i think im making a mistake where im trying to adjust fuel then timing then fuel and i got the mixture so far off that its killing spark... only 3 cylinders seem to be giving strong spark and it was good before. so im dealing with weak spark that is causing my issue.. i got it much better after replacing the 3 coilpacks but the result was short lived and i definitely have a spark issue.

im not sure why my 3000gt is giving me such issue... i dont have this problem with other cars i worked on

Don't forget to check the coil to plug leads for their resistances, if the coils aren't directly fitted to the spark plugs - not too familiar with the engine - or between the coil output and the plug terminal, as some coil-on-plug designs have built in resistors that can fail.

There are exceptions, but you will usually get around 10-20k Ohm per lead.

Oh, check to see if the engine is supposed to use resistor spark plugs, as using the wrong type can also cause problems, as can the wrong plug gap.