Intermittent poor starting?

[KBB]

Changing the O2 sensor will make no difference to quicker starting, the ECU will only go into closed loop control after the sensor has heated up around 2-3 minutes after the engine has started even with O2 sensors with a heater circuit.

Longer cranking times are also a symptom of a failed Camshaft positioning sensor or loss of signal. Without a cam positioning signal the ECU cannot conduct cylinder ID necessary for sequential fuel injection so reverts to bank firing all injectors once every engine full cycle (720 Deg or two full turns of the crank) hence the delay in firing.

Contrary to popular belief the ECU will not always throw a DTC for a failed CMP sensor.

[sarumboy]

Thanks for that KBB. Initially the reason for changing the O2 pre-cat sensor was to try and prevent the 0171 code keep coming up. It was said on here that might be the reason why it does. At least its eliminated now.

For the poor starting, you've convinced me next on my list should be the camshaft sensor. At this moment, I don't even know where that is, but I'll find it and change it! Thanks.

[KBB]

The problem I see on a lot of forums such as this is that the owner wants to repair the vehicle on the cheap and avoid the garage bills and I understand this.
I also see people with code readers retrieving a fault code and then go straight for a component change. This is where the untrained enter the minefield of repair by substitution of components which inevitably ends up being more expensive than a trip to a garage.

In the world of diagnostics technician's are trained to treat the fault code as a symptom and not the cause. The DTC P0171 "System too lean" is a classic for which I have seen the O2 sensor, MAP/MAF, fuel filter and fuel pump changed to name but a few and the fault remain only to find the fault was a simple vacuum leak.

If you have a decent DVM that has frequency or duty cycle on it you can test if the CMP is producing a signal. The testing should be done at the ECU end and not the sensor.

[morrismen]

Agree with KBB. Often poor initial starting is either loss of vacuum or limited air supply. Simple things to check, even replacing the air filter or looking at the hose connections on the air intakes and air filter box can make a difference.

[KBB]

When I'm looking at a P0171 DTC best approach is with a scan tool that supports live data and freeze-frame data. Freeze-frame will give key info as to the engine condition and load when the DTC is set.

Next look at the STFT and LTFT at idle and then part load, this will provide further key info for the diagnostic path.

Final test would be to do a snap throttle test whilst graphing O2 senso1 bank 1, TPS and MAP/MAF, this will provide key info on the ECU fueling strategy.

[960kg]

Originally Posted by KBB

When I'm looking at a P0171 DTC best approach is with a scan tool that supports live data and freeze-frame data. Freeze-frame will give key info as to the engine condition and load when the DTC is set.

Next look at the STFT and LTFT at idle and then part load, this will provide further key info for the diagnostic path.

Final test would be to do a snap throttle test whilst graphing O2 senso1 bank 1, TPS and MAP/MAF, this will provide key info on the ECU fueling strategy.

Of course you are correct.....

But owners are here because they want a fix for there motor.

The majority who post are not experienced so that is why they post!

So you are telling them to spend hard cash on the tools to read what you stated and then when they cant` find the problem or don`t understand how it all works then they are no further down the road and still have to spend to fix the problem.

So why not go to the garage in the first instance get it repaired pronto for a given sum and your on the road again.

Or take a chance and buy the part the error code points to!

To purchase expensive gear for readings error codes in certain ways is surely not really cost effective with out most reliable Volvo`s.

Don`t get me wrong, you are correct what you say, but then you would be as it is your job!!

[KBB]

The is defiantly a need for a balanced approach, buying a low end code reader from ebay dose not make you an automotive master technician. The are many examples where an OP is bombarded with advice leading him down a rout of fit this, replace that and 300 pounds later no fix.

The old saying "a little bit of knowledge can be dangerous" is often the route of miss diagnosis resulting in a timely and costly approach. I am fully aware that in tight economic times people want a cheap fix and are horrified at the hourly rates charged for diagnostics and repair work, but this is not a cheap industry to operate in. Equipment, training, access to current technical information and the other overheads comes at a huge price(more so if you cover multi makes) which is reflected in the hourly rates.

I agree as with all trades in this life the are poor operators, average and very good no different to builders, plumber's, electricians ect.

On the topic of scan tool's, a couple of hundred quid would get you a decent OBD2 compliant reader with freeze-frame & live data and rechord capability, as ever you get what you pay for. As for taking a chance on changing a component based on a DTC would you be happy if the garage took that approach?

It is not may aim to start an argument but merely look at both sides of the issue, some have got lucky based on probability and others have spent a lot and got no where fast. I have not got a problem with offering diagnostic advice on this forum at all.

[brynmor530]

I agree with what KBB says about replacing parts purely as a result of an error code, which is why I prefer the 'remove, inspect, clean, test or have tested where possible, fix if possible and necessary and refit' approach to see if the problem can be solved rather than going for immediate replacement.

Obviously if you have a code reader and can use it effectively your are at an advantage but I have a friendly local garage who will do that for me for a few of beer tokens so I at least have a starting point. As KBB says the codes are there to display a symptom not necessarily the root cause.

My limited experience is with classic cars and as such the basics of what cause running issues are always the first I check such as vacuum leaks, fuel delivery, electrical connections, weak spark, timing etc and even compression. I've saved a lot of money with this approach over the years with (as far as the engine is concerned) the FPR being the first component I will have replaced outright on the Volvo, (garage where I bought the car from replaced Cam Sensor which didn't fix the fault) having checked and found fuel leaking through the diaphragm of this component. I am however acutely aware that this may not be the only cause of the cars slow starting as often (with classic cars) problems like this are caused by more than one thing being worn or set up incorrectly and so that remains my mindset when working on a more modern vehicle.

Symptoms can be the result of multiple issues culminating in the problem to be solved and so what I'm trying to say is, the approach to fixing them should always be sequential and since the point of doing this sort of thing yourself is to save money, it makes sense to start with the cheapest option first, which brings me back to, remove, inspect, clean, test or have tested where possible, fix if possible and required and refit.

Having said all that I find forums such as these invaluable as peoples experience can often point you towards a solution which you may not have considered otherwise and the technical input from the likes of KBB, which can be expensive when given face to face as part of a diagnostics report, is a great help to keeping our cars on the road at a reasonable cost.

I for one hope that the questions continue to be asked and the responses continue to be given by everybody of all abilities.

[960kg]

At last we have a sensible discussion rather than the berks picking holes in what owners post.

This is where the problems always arise if someone suggests a way or an answer to a problem then it can be ridiculed by someone that feels there toe`s have been trodden on or think they know better.

I for one have always been treated this way with most of my posts but at least i do try to help where the accuser will disappear and leave the OP stranded as what to do next. Sorry , but this is not my way, members should always try to live and let live despite the way words are written. It is very difficult to get in the same groove of the offender.

I have always stated that my posts are only my own opinion and do not take umbridge to any other help given to the Op but i do resent rudeness which i always will reply to with equal rudeness.

We are all different with different issues and we know about certain areas of our motors more than others. I am the mechanical side of things which i have learnt since Kingdom come. Ours motors may all be computer controlled but what they control has been the making of all engines since they were produced and is still the same today as it`s always been.

So i do agree with what has all been said above and hope all do to continue to post but more than anything hope that members will give and take even if it is not what they like seeing!.......there is more than one way of skinning a cat....

[skyship007]

I'm not going to buy an expensive code reader for my non OBD2 compliant 2003 diesel because they seem to have a habit at pointing at one system or item only for the fault to turn out to be the next one connected to it, so I would not buy an expensive new part based on that type of info. I might risk buying a scrappers part that might be a useful spare if it turned out to be a connector or water in the box etc.
In a number of cases the cheap code readers have a habit of telling you what you knew in the first place, once you factor in the adjacent components. If I get a fault, although I would like to know the code, I tend to look at the statistics first in terms of the most likely culprit and see if that can be checked without buying a spare.