Classic Misfiring at 2-3000rpm

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Looks like another day on the spanners Adam I'll check n see when I'm off and pop down when the new reader arrives looks like a fueling issue for sure now.......what exactly is the mystery
 
Hi All.

Got my diagnostics got the 14CUX through loaded it up and had a look.

No fault codes. Disconnected a few things to throw fault codes and they came up. Cleared them.
So.. the misfire. It seems we have the O2 sensors locking out and these are the new ones.

I'll post a video shortly
 
ukadamwest said:
Hi All.

Got my diagnostics got the 14CUX through loaded it up and had a look.

No fault codes. Disconnected a few things to throw fault codes and they came up. Cleared them.
So.. the misfire. It seems we have the O2 sensors locking out and these are the new ones.

I'll post a video shortly
Click to expand...

Is there a chance the replaced O2 sensors came into contact with grease? Some folks use it to lubricate the threads to allow easy screwing in and out. However, due to the extreme temperatures involved, it is a waste of time as the grease burns up and it makes no difference... I'm sure you kow.

Anyway, there is another small possibility that the temp sensor is working outside parameters and is unable to send the correct signal, leaving the engine open loop running as both sets of sensors rely on a signal from one another to "advise" the ECU when to switch and how long for.

So, if you're sure the sparks are there, fuel is getting delivered when the engine is cranking, then it has to lie with the signals from the sensors. The O2 sensors wont play until they start heating up and "sniffing" for O2 since a cold start needs extra fuel anyway.

This takes us to the temp sensor, it has to decide if it's cold, warm or hot. If it's cold, we are open loop (no O2 signal). If it's perceived as cold (faulty temp sensor) the ECU still wants to run open loop, rich mix. Same if it's hot and "thinks" it's still cold.

So, either, the temp sensor is faulty or the wiring is faulty, stopping a signal reaching the ECU or the ECU is faulty! I believe you have tried an ECU. If "it" then "decided" to run open loop, the engine should still start cold, but would run like a bag of spanners, taking us back to a faulty signal....

Now you replaced the O2 sensors and as far as we can guess, they were ok, but were they? What if the sensors were faulty, what if the plugs on them were faulty or the wiring from there to the ECU? Mine had been bodged before I got it. I simply removed them, binned them and replaced them with genuine Bosche jobs. Lucky for me, the two rich problems were obvious, mice ate 3 of the leads on one bank and the soldered wire on the O2 sensor was on the same bank.

Best I can do at the moment.
 
I meant to ask, is your car pre or post 96? It's just possible that if it's early enough, it may only have non heated O2 sensors, post 96 should be heated sensors. If they are heated, check they are heating up during the running cycle. if not, this too can trigger a fault.

Another possibility is if the sensor is originally old type, did the originals have a hole on the sensor for it to "breath" through? Newer sensors are able to breath through the wiring. So, if your car needs older sensors, but has been upgraded, there is a small, but significant chance they are unable to perform within parameters for your car.
 
Hi Adam, just read through it all. I have had the same problem years ago. Can't remember what it was though :p
Are you getting fuel to the rail now?
What's the pressure like?
I definitely had a failed pump once, other than that I was going to say spark tracking but wammers beat me to it.

You could bin the o2 sensors and change tune rwsistor. There is no requirement pre 93
 
Perhaps it's just the way the video is jumping about, but the lambda should be flip flopping between less & more, but it's lean then nothing then rich for a long time.
 
Thanks cooltide, i did start to think about a decat. if it is easy as swapping out a tune resistor, then it would make sense to have one spare as a trouble shooting aid. i want to keep it as close to factory spec as possible and yes, with a 1990 there is no requirement for cats.

I'm getting the right MAF reading, no fault codes, correct temp sensor readings, strong spark it idles fine as will be noted from the video link when the engine starts it idels fine, when you blip the throttle the O2 sensors go full 100% and stay there with no variation.

with two different fuel ECU's two different MAFs all tried in various combinations it gives the same result. ergo i can only surmise that the O2 sensors are not functioning correctly.
 
Doo said:
Perhaps it's just the way the video is jumping about, but the lambda should be flip flopping between less & more, but it's lean then nothing then rich for a long time.
Click to expand...


you got it Doo. i left it idling for 2 minutes and blipped the throttle during that the lambda stay 100% rich.

when it starts and idles the Lambda readings stay central, as soon as the throttle is blipped it goes full rich and stays there.
 
Atch..

just had a thought after some sleep from a hectic week. The sensors read what they are *reading* not what they are instructing, so they are reading rich because that is what they are sensing, correctly.

so, as a rehash.. O2 sensors good and reading the rich running, equally across both banks. MAF good, ECU good, temp sensors good, ignition all good

but overfueling.. as myfirstl322 suggests above.. FPR which has been nagging in my mind for the past week while ive been waiting for the diagnostics to arrive
 
indeed both ECU's gave the same result. but all sensor readings are within parameters, except for O2 which are reading too rich.

so in my mind, we have too much fuel being pushed through, the pump pumps and the FPR regulates the pressureand is mechanical with electonic input. It is, in my mind confirmed by the response on the diagnostics, the O2 sensors read neutral on start and idle, but when the throttle is moved, the O2 readings creep up and stay up, which would suggest that its the FPR diaphragm or the inlet manifold vacuum off of it but the vacuum pipe is ok

but i could be wrong
 
Last edited:
The O2 sensors are reading rich because it's running rich, no?

But it doesn't "think" It's running rich.

So, the ecu doesn't compensate ... Or does it? What would show you the ECU trying to lean down the mix?

Is it even possible, all sensors are correct, but the fuelling is being controlled by an outside force such as a vacuum leak, causing the ecu to "have" to up the fuel mix.

Plenty places to hide as there are several vacuum operated devices on these, any of which could be leaking air.
 
i would agree.. the sensors reading rich, because its running rich, not that they are faulty..

i would agree again that it doesn't think its running rich too, doo.

so the fuel, goes from the tank to the fuel rail (and injectors), by way of pump, the pressure of which regulated by the FPR.

With two different fuel ECU's, Two different MAF's and two different sets of 02 sensors giving same result, overfuelling, yes i agree, it can only be either a vacuum leak between the plenum and FPR (ruled that out earlier) or the FPR itself .. would you agree ?

an alternative could be the throttle pot
 
Last edited:
ukadamwest said:
i would agree.. the sensors reading rich, because its running rich, not that they are faulty..

i would agree again that it doesn't think its running rich too, doo.

so the fuel, goes from the tank to the fuel rail (and injectors), by way of pump, the pressure of which regulated by the FPR.

With two different fuel ECU's, Two different MAF's and two different sets of 02 sensors giving same result, overfuelling, yes i agree, it can only be either a vacuum leak between the plenum and FPR (ruled that out earlier) or the FPR itself .. would you agree ?
Click to expand...

Only one way to find out...;)

Don't rule out any other possible vacuum leaks though. Have you tried spraying Plus gas or WD40 or some such around the manifolds and associated pipes? Start the engine, let it idle & check the O2 mix is lean, then start spraying and watch the computer readout. If it goes up without revving, you may find you have a vacuum leak.
 
Methinks me found the problem...

20170205_142953_zpsvsxlkrmk.jpg
 
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