EFI diagnosis techniques

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EFI diagnosis techniques

Postby Yoshi » Thu Feb 26, 2009 9:30 pm

Table of Contents
Preface
Intro
Section 1: Air
Section 2: Fuel
Section 3: Spark
Section 4: Inputs to your ECU
Conclusion

*Edit: Some people suggest checking spark first since buying a spark tester is very quick to test and to get out of the way. See post http://www.e30tech.com/forum/showpost.php?p=701687&postcount=18


Preface: I am not an EFI expert. I have read many books and I'm probably wrong on a few things so help me fix this up! Second, whole books could easily be written on these topics so things are condensed for the average home mechanic as best as possible.

Intro
The first step into understanding why your car isn't running right is what goes into it. Your car measures an exact amount of three things: Air, Fuel, and Spark. A bad amount of any one of these will easily make your car run badly or not at all. I have broken the next sections into these three ingredients. When diagnosing any particular problem, I always mentally go down this list in random order and you should too.

I have bolded main headings and sections. Key words are italicized for you to quickly search for the component you are currently diagnosing.

*Edit: I have also included a section on ECU inputs as to not clutter up the Fuel/Spark sections

Section 1: Air
Unlike fuel and spark, your car can not measure out a specific amount of air to be put into your engine. The air basically flows in and it happens to try to measure it as it goes past. Because of this fact, air is the most likely cause of any previously running car to studder/idle high/die/not start. The sole thing in your car that measures air is that little box attached to your ebay cone filter or stock air box.

For the purpose of this article, you dont need to know how it works, just that it counts/measures air as it goes by. This leads us to the conclusion that if air gets in (or out) after this point, that box has no idea. Too much air can cause a car to buck while driving, idle higher than it normally would, and of course, not start. The unmetered air getting in after the AFM (air box) is called a vacuum leak.

First, if you have messed with anything recently, make sure the hose clamps and bolts for everything intake related are bolted down. If gaskets are bad or the hose clamps arent tight, air will almost always get in unmetered. Also make sure your rubber intake elbow has no cracks or tears. You may think that ductape is holding well but it will not hold a good vacuum. The most commonly missed vacuum leak is the evaporator canister. Part #20 in this picture (http://www.realoem.com/bmw/showparts.do?model=1113&mospid=47309&btnr=16_0107&hg=16&fg=10) attatches behind the TPS (#24) in this picture (http://www.realoem.com/bmw/showparts.do?model=1413&mospid=47322&btnr=13_0335&hg=13&fg=15). Usually that hose (previously mentioned part #20) cracks and leaks air. Since it is in the hardest to find place, it easily goes unnoticed.

The last major item is the ICV (idle control valve). Upon failure, the car should start but might take some cranking and will idle higher and maybe waver around. The quickest test is to unplug the wires while the car is running. If there is no change, it could be it and further testing is needed. If it does change, then it works.

The rest of the vacuum/air system consists of a vacuum hose that goes to the fuel pressure regulator, crank case to throttle body vent tube, and the brake booster lines. Simply make sure everything is connected and AIR TIGHT. Brake booster lines tend to wear out and slip in and out easily, just use some RTV to seal it and that leak should stop. Small leaks will not effect idling/driving/starting but will kill gas mileage.

Summation, cracks/tears are unacceptable. Replace, and do not patch.

Section 2: Fuel
Fuel is trickier than air. Your ECU properly calculates the amount of fuel to be put in with given inputs. If these inputs are bad, both fuel and spark will be off. Checking these are in Section 4.

Anyway, your car's ECU injects an exact amount of fuel simply by opening your fuel injectors for milliseconds at a time. This takes into account MANY assumptions. For this section we are assuming the ECU is properly calculating that millisecond amount and that the other assumptions are wrong.

The first major assumption is that the signal for the injectors to open or close is getting to the injectors. The stock Motronics box fires fuel in 2 batches (3 injectors at a time for M20s (you heard me right)). If the wiring is bad, only half the cylinders will get fuel. This will cause the car not to run AT ALL. I have yet to see an M20 run on less than 4 cylinders but you can prove me wrong. Fixing this problem requires finding where the wires are shorting and replacing that area of wire. Diagnostics of wiring are beyond this article.

The rest of the fuel system can be diagnosed with my favorite tool. The in-line fuel pressure gauge. It is easily installed in the fuel lines on the feed line before the fuel rail (do not hook it up to the FPR, you want the end that goes to the rail itself). IIRC the stock FPR on an M20 is 3 bar. Most FPRs have their pressure rating written on the side or dimpled in. 3 bar is about 43.5psi. If your fuel pressure gauge reads about that at idle, congrats, we know what the problem isn't!

If your fuel pressure correct:
The only other part of your fuel system that could be bad at this point is your fuel injectors. They could be dirty, clogged, or dead. To test a dead injector (warning, this is kinda dangerous) take a long flat tip screwdriver and press the tip on the center of an injector. Put the other end to your ear and listen while the car runs (or cranks). You should hear a pronounced *click*. Repeat for all injectors. If they all click, they are all working.

Dirty injectors will "spit" fuel instead of spraying a fine mist. Gasoline ignites best when properly atomized (misty goodness). Droplets will either combust weakly or not at all. Unfortunately, diagnosing this is near impossible without proper tools and is more than I know. The easiest solution that might (read: probably will help a tad but not much) is fuel injector cleaner. If your idle improves then you might be onto something. If not, you still can't rule them out. The BEST option is to send the injectors to be flow tested and professionally cleaned. It will run you about $100+ (I think I got quoted $200 with shipping) but you will get back a sheet of paper that tells you what they were flowing at and what they are now. You will be surprised at how uneven your injectors are. Note: dirty injectors generally will not keep a car from starting.

If your fuel pressure is under what it should be:
Again, the fuel system is very troublesome. The fuel pressure gauge just showed you a very general area of problems but we now know about where it is! Here is the general fuel system:
http://www.realoem.com/bmw/showparts.do?model=1113&mospid=47309&btnr=16_0107&hg=16&fg=10

The parts we are most interested in are the fuel pump, fuel filter, fuel pressure regulator, and the lines in general. If you are getting 0psi fuel pressure, the first thing we want to check is the fuel pump. You can use the same screw driver method against the fuel pump (under the passenger's side rear seat cushion) to hear if it's running. You can also crank and check for 12v to the fuel pump. A few times mine read 5v but I think that was due to a bad connection. If you get less than 12v, it might be some wiring/grounding. If you get 0v then it could be your fuel pump relay or dead wiring. (anyone else know what to do in this situation?).

Note: NEVER EVER EVER EVER run your fuel pump outside of the fuel tank. You could permanently damage or destroy the pump. Ask me how I know? (poor walbro 255).

If the fuel pump turns on and we are getting low fuel pressure, there is a chance there is fuel leaking out from the coupler between the pump and the metal pump housing. Pull out the housing and check for broken or leaking anything. You can replace the old plastic donut retainer with BMW in-tank fuel lines. Regular fuel hose is not meant to be exposed to fuel on the outside and will swell and burst. Again, ask me how I know. While the pump is out, check the filter screen and inside the tank for debris. Floating rust and other objects will cover this screen up and could prevent the pump from getting enough fuel. (This has happened to me too...)

Low pressure can also be caused by clogs. Some people will remove their hard lines and clean them but I just replace all of my soft (rubber) lines. If your fuel filter has not been replaced and even if it has, changing it out might solve the problem.

The last thing to check in the system is the fuel pressure regulator (FPR). It is two 10mm bolts and is attached to the fuel rail. The only decent way of testing this that I have found is to find a spare and replace it. But I have yet to see one actually fail (reply to this thread if you have).

If you have gone through all of these steps and you have no leaks in your system but are sure its fuel related, a full cleaning of your fuel system might be in order.

Section 3: Spark
Spark is probably the easier of the 3 to test. But to start, if you have not replace the following: Distributor cap/rotor, ignition wires, and spark plugs. This will cost you around $200 for an M20 ($140-160 for the wires iirc) but here is why you NEED to replace them.

Dist cap/rotor: The contact points will wear, some will chip off. This means one or more of your cylinders will not get the same spark as the others if at all. Replacing only one will only make the problem worse or better for a short while.

Ignition wires: Old wires will degrade and slowly start to internally arc. This will give inefficient spark and could lead to fouled plugs. This is expensive but I have seen many cars run MUCH better after just this.

Spark plugs: Properly gapped and fresh plugs (I use copper and change them every 2 oil changes) will spark cleanly and ignite the fuel/air mixture best. dirty plugs might not even spark at all. They are so cheap, there is no excuse not to even try this.

I did not mention the distributor coil simply because it is either a work or not work item. They rarely die but we will diagnose if that is the problem very quickly. To test if your ignition system works do the following:

*WARNING this is a not really approved way of checking for spark. I am not responsible for you blowing up your garage because you like to spill gas everywhere!*

Pull out one of your spark plug wires. Find a thick gauge wire aprox 12"+ long. Put one end into the plug wire so that it is inside the contact point. Hold the other end close to but not touching the valve cover. If your valve cover is painted, go for something not painted and conductive like the shock tower ground on the passenger's side. The intake manifold is bad because it might not be grounded properly. Have your friend try to start the car (btw dont do this while the car is running. I dont know how you would pull a plug wire while its running tho...) and vary the distance of the wire to the grounding point. If you see a spark, yay you have spark. If not, you either did it wrong or you arent getting spark. Repeat for ALL cylinders.

If only certain cylinders get spark, your dist cap/rotor might be bad or one of the ignition wires might be bad. If no cylinders get spark, your coil might be dead or the ECU isn't sending a signal to the ignition system.

Here is a better, more accepted way of diagnosing spark
Image
ark plug tester is your #1 asset when diagnosing ingition problems, coupled with a decent DVOM (voltmeter). Spark tester is again like $15, and no need to break the bank on a DVOM, but having even a $50 one is priceless. An LED test light rather than a conventional filiment bulbed test light probe is priceless as well when dealing with ignition coils and other pulsed items, as a DVOm will simply read 6/7v instead of fluctuating form 0-12v like the load really is, because it is pulsing too quickly. And a filiment bulb will merely glow bright/not quite as bright, while and LED bulb can flicker on and off at a very high rate.

Another quick check can be to check out your ignition components while running the car at night, it's an easy way to see when your plug wires are fucked, coz at night they will corona and glow blue streaks as they fire quite brightly... you'll know it when you see it.
If you have replaced all of what I mentioned and you still arent getting spark to some/all of your cylinders, you have a deeper problem.

Section 4: Inputs to your ECU
This section is tricky because diagnosing faulty sensors is very hard. If the sensors within your car are either not reading right or reading at all, the spark and fuel outputs will easily be off. This could cause anything from hesitation, bad idling all the way to not starting at all. Since there arent any real readouts of these sensors, figuring out whats wrong is very hard and I have almost never done it on a stock E30.

The first major thing to check is the AFM. I have no idea how to do this... anyone want to chime in so I can update this part?

Believe it or not, grounding greatly affects your car. If the power given to the components of the car can not ground well enough, your car might not even turn over. Be sure to replace the engine ground strap that goes from the oil pan to the chassis on the driver's side of the car. Check everywhere for frayed or damaged wires and replace them. Also be sure to check the battery voltage to make sure it has enough to start the car.

Other sensors to check/replace include the TPS, coolant temp sensor, and others. The TPS will not keep the car from starting but will cause hesitation or cutting out when you try to accelerate.

*This section could use more info on how to test sensors. Help me out by giving suggestions*

Conclusion
This article is a glaring generality of the EFI system and how to diagnose it. There are much more in depth ways of checking everything but this article should either solve your problems or give you enough info to pinpoint where or about where your problem is.

If you have comments/suggestions/info to add please reply to this post.
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billyconnite
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Re: EFI diagnosis techniques

Postby billyconnite » Fri Nov 05, 2010 8:43 pm

Hi there, nice write up. Regarding an FPR:
The last thing to check in the system is the fuel pressure regulator (FPR). It is two 10mm bolts and is attached to the fuel rail. The only decent way of testing this that I have found is to find a spare and replace it. But I have yet to see one actually fail (reply to this thread if you have).

It happens. There's 2 easy ways of testing, generally fuel will start leaking through the vacuum line on the FPR, and there can be quite a bit.

To test; Replace the black stock vacuum line running from the FPR with a clear hose, if you see fuel in the line when the cars running, you need a new FPR. If you don't have some hose lying around, remove the vacuum line, and get a friend to hold his thumbs on both plugs (Won't start if you don't i believe). If you don't have a friend lying around, you may need to find one of those. Start the car, let it run for a bit, turn it off. Remove thumbs. Fuel should squirt out of the FPR if the FPR diaphragm is broken.

This won't cause a no-start, but will cause excess fuel to run back into the intake manifold, possibly causing the engine to:
    Run rich.
    Miss a bit (too much fuel in the cylinder).
    Hard starting.
    Bad gas mileage.

-Rhett.
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Re: EFI diagnosis techniques

Postby Yoshi » Fri Nov 05, 2010 8:45 pm

good info. Thanks for the input!
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Re: EFI diagnosis techniques

Postby M20_fever » Fri Nov 05, 2010 9:31 pm

great first post!
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Re: EFI diagnosis techniques

Postby bmwjac » Tue Dec 28, 2010 11:55 pm

What is a TPS and where do i find it?
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Re: EFI diagnosis techniques

Postby Yoshi » Wed Dec 29, 2010 12:10 am

throttle position sensor. It's under the throttle body.
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Re: EFI diagnosis techniques

Postby bmwjac » Wed Dec 29, 2010 3:51 am

thank you , that might be the solution to my problem
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Re: EFI diagnosis techniques

Postby bmwjac » Wed Dec 29, 2010 4:08 am

why would there be an oily residue inside the air intake? The same residue is covering the TPS
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Re: EFI diagnosis techniques

Postby Yoshi » Wed Dec 29, 2010 1:25 pm

Please start a new thread on your specific problem so we can help you better.

The short answer is that there's a PCV pipe from your valve cover. Youre getting a bit of oily residue from the valve cover into your intake. It's fairly normal. Just clean it and go on your marry way.
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Re: EFI diagnosis techniques

Postby bmwjac » Wed Dec 29, 2010 3:39 pm

ok thanks

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