ST185RC poor running below 2500rpm

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ST185RC poor running below 2500rpm

Postby thetyrant » Thu Jun 28, 2018 8:04 am

Ok finally got my ST185RC on the road for summer and while it runs very smooth above 2500rpm it struggles with anything other than very light throttle under that RPM, if you let it gently accelerate from 1000rpm upwards it feels "reasonable" but its like something switches around 2500 and runs much nicer, if you try to give it more acceleration below 2500 it doesn't like it at all if you accelerate too much.

My first thought was maybe EGR as I believe that switches around then but then I found the JDM and RC models etc don't have EGR so that rules that out! TVIS is next to check to make sure its closing at lower rpm but not sure It would give the noticeable step at 2500rpm im feeling!

Ive been servicing it up and new oil, filters and basic check over but nothing obvious wronf, found 1 Vac hose starting to split but don't think it was actually leaking as yet, will check further in when I im looking over TVIS.

Any ideas welcome :D

Thanks
Ian
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Re: ST185RC poor running below 2500rpm

Postby underscore » Thu Jun 28, 2018 6:15 pm

Sounds like an intake or exhaust leak to me, I'd do a quick boost leak test.
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Re: ST185RC poor running below 2500rpm

Postby thetyrant » Fri Jun 29, 2018 5:32 am

I thought a vac/boost leak as well but not been able to find any as yet!..... anyhows problem seems to be fuelling related and more specifically the dual voltage fuel pump feed via the fuel pump relay, another GT4 owner told me that 2500rpm is about the time the fuel pump switches over to full voltage from the low voltage supply it runs below that, you can fit a jumper in the Diagnostic box between B+ and FP and this gives fuel pump full voltage at all times, tried this and car is now perfectly smooth at all revs/loads :D

Will run with the Jumper in place for awhile make sure problem stays away, also will check the relay for function as maybe its corroded or worn etc meaning the low voltage is too low, would be nice to keep it all original and retain the duel voltage feeds but if I need to run full voltage at all time its not a problem and always did that in my old Evos anyhows.
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Re: ST185RC poor running below 2500rpm

Postby underscore » Fri Jun 29, 2018 7:23 pm

If the relay is acting up you can bypass it and leave the rest of the circuit intact. There's a diagram on here or you can try to find the OEM jumper that was in some 2WD 5th gens.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
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Re: ST185RC poor running below 2500rpm

Postby thetyrant » Sun Jul 01, 2018 10:12 am

If got it bypassed and running nicely by fitting a wire jumper in diagnostic box between B+ and FP, but would like to work out why it doesn't work correctly if I can, im suspecting the resistor or wiring to it is at fault and dropping volts to low maybe but will check into things further.
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Re: ST185RC poor running below 2500rpm

Postby Magroo » Sun Jul 01, 2018 2:13 pm

My car was acting similar when I found out a few of the capacitors had died in the ecu leaking all over the place. Its a small pain to get to the ecu but might be worth checking.
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Re: ST185RC poor running below 2500rpm

Postby thetyrant » Sun Jul 01, 2018 4:02 pm

Magroo wrote:My car was acting similar when I found out a few of the capacitors had died in the ecu leaking all over the place. Its a small pain to get to the ecu but might be worth checking.


Interesting where is ECU on these? I will maybe pop it out to double check all is ok in there, although it runs well now with pump getting full volts, apart from when cold its still a bit lumpy.
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Re: ST185RC poor running below 2500rpm

Postby Magroo » Sun Jul 01, 2018 5:08 pm

Pull back the rug in the passenger foot well near the center dash area. You should be able to see the ecu plugs. I think there are 3 mounting points. Have to do the same on drivers side to get to the nuts that hold it down.
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Re: ST185RC poor running below 2500rpm

Postby thetyrant » Mon Jul 02, 2018 7:27 am

Spot on thanks same place as my old Rav4 then, will pop it out for a check over.

Cheers
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Re: ST185RC poor running below 2500rpm

Postby thetyrant » Thu Sep 13, 2018 9:09 am

Just to update this, car hasnt had a lot of use only the odd sunny days ride out but this week ive had to press it into daily duty as my bmw is in the shop for some work, anyhows the Celica was running ok for a couple of days and then the poor running at low revs returned!

First thing i did was make sure the jumper in Diag port i had fitted was secure and it must of worked itself loose or started to make poor connection to terminals as once i removed it and refitted it all was well again, ive been meaning to check the actual resistor and make a better bypass setup so i set about finding it all last night, i wanted to be sure it was the actual resistor at fault and not some other issue that jumping diag port was temporary fixing, also wanted to free up diag port and leave FP relay in its place etc.

After some searching online i found the resistor was located under the battery fuse box area so removed battery last night to get access to the area, there are 2 resistor packs on the bracket (not sure what other one is ? has lot more wires attached) but as i knew the FPR Resistor was 2 wires unit going from manual i unplugged and tested that one, sure enough resistance was 1.6ohm nearly double the 0.7ohm specified in manual so 100% the cause of low fuel pump volts/pressure and poor running at low revs.

I let it unplugged and started car which to my suprise it did and wondered if i had got correct unit, however after a few seconds car dies so im guessing it uses full voltage for inital startup before switching to low voltage once running and idling, then i bridged the connector that is normally plugged onto resistor and it kept running fine for short testdrive so all good, ive left that bridge in place but will probably cut connector and join wires with solder for permanent repair.

Now i need to work on the creak from front suspension :D
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