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Greg, Technician
Category: Car
Satisfied Customers: 1129
Experience:  Automotive diagnostic tech
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Proton Persona 1.6GLi, 1994. And going strong until this.

Customer Question

Proton Persona 1.6GLi, 1994. And going strong until this. Failed MoT emissions at low speed idle with very high HC. I replaced the Cat, the Oxygen sensor and the front exhaust pipe (suspect tiny leaks). Recently serviced, oil and all filters, plugs, etc. As it comes in to the test station after a good run, the HC is immediately tested and initially ok, but then just slowly creeps up and up to 300+ppm on slow idle and 230+ on fast idle. Local experts think that something is telling the ECU something wrong and causes it to richen the mixture, ie widen the pulse width, so its runs far too rich. Given the parts replaced, is it reasonable to suppose the ECU has a fault? The only thing I can think of to explain the behaviour is that the ADC has a fault on the low order bits (probably 10 bit ADC) which are insignificant at high air flows, but critical when idling. Any thoughts? Philip ***********

Submitted: 2 years ago.
Category: Car
Expert:  Greg replied 2 years ago.
Hi Phillip . Do you have any malfunction codes .Can you carry out a fuel pressure leak down test, bad/weak injectors can cause this .Greg
Customer: replied 2 years ago.

When last on the diagnostic, with this problem, there were no fault codes. The injectors have been cleaned on an ASNU system and do not appear to be leaking when de-energised and fuel pressure seems to be about 40psi when the reg is connected to the inlet manifold and about 50psi when not, spray patterns appear to be ok.

Expert:  Greg replied 2 years ago.
Hi Phillip .
And after driving a few days no codes .
What are the fuel trims after sat a few minutes at idle .
Customer: replied 2 years ago.

If by fuel trims you mean the emission results, I have those, after a few minutes at idle, they are:

Fast Idle Test

CO 0.58%vol Fail against 0.30

HC 296ppm Fail against 200

Lambda 1.054 Fail against 0.980 to 1.020

Second Fast Idle Test

CO 0.75% Fail against 0.30

HC 308ppm Failed against 200

Lambda1.030 Failed against 0.980 to 1.020

Natural Idle Test

CO 0.020% Pass Against 0.75

HC 123 ppm Pass Against 200

Lamba 1.022 Fail against 1.020 to 0.980

If fuel trims means something else, please explain and I'll try to get them.



Expert:  Greg replied 2 years ago.
Hi Phillip .

Fuel trim is in the scan tools engine live data .

Fuel trim will let us know if you are running to rich or to lean .
Will look like this in the data stream :STFT1 and STFT2

Readings should be done at idle and at 2500rpm.

Need to know if you have a negative or positive reading will help us narrow down the problem area .

This is the best way to diagnose when you have no malfunction codes .

Customer: replied 2 years ago.

OK. I'll try and find someone who can do this without breaking the bank. Meantime, do we just suspend this conversation until I can get back to you?

Expert:  Greg replied 2 years ago.
Hi Phillip .

Will keep this open .until you have the data, no problem ,just keep coming back tpo this page .

Customer: replied 2 years ago.

Proton cars (UK) tell me that this car does not produce the STFT1 and STFT2 information, they came in later models.

Expert:  Greg replied 2 years ago.
This is what i suspected, but some have the obdii the very late 94

I wanted this to know if you was lean which would cause you this issue ,did you check for
intake vacuum leaks ,small leak at the throttle body or intake manifold. can you have this checked .
Or if you have a vacuum gauge hooked up to the intake this could also help .

Here are possible cause for this issue you have if the vacuum is not leaking .

MAP.this is a common issue
O2s.this is a common issue
TPS.Check adjustment
IAC ,this is a common issue

If no codes and no vacuum then the readings of the above should be checked and repaired/adjusted .

Rgds Greg