Volkswagen Passat Forum banner

1 - 20 of 28 Posts

·
Registered
2003 Passat GLS 1.8T MT
Joined
·
16 Posts
Discussion Starter · #1 ·
I've read that fuel pumps used in B5 Passats (and others) can be tested by checking resistance at pins 1 and 4, with the stated tolerance being 1-3 ohms. Most videos/posting show healthy pumps yielding values around 2.5 ohms. My question is the following: If a pump shows resistance at the low end of the tolerance range, say 1.2 ohms, is this an indication that the pump is "technically functional" but on the way out. In other words, is the resistance test comparable to how you can use voltage to check the relative health of a battery.

For background, I've been working on a LEAN fuel trim issue with my 2003 Passat 1.8T (AWM) and pretty much narrowed the problem to fuel delivery. Fuel trims are fine at idle but go positive under load (driving). I've ruled out vac leaks, injectors, and fuel filter, so now looking at the fuel pump as a source. I realize that checking fuel pressure would be helpful, and I plan to. But I don't have the tools at the moment, so for now checking the easy stuff like fuel pump resistance. Thanks!
 

·
Registered
Joined
·
1,481 Posts
It's possible a low resistance means that the motor coil is shorted and so the motor current that is supposed to go through the entire coil is only going through part of the coil, thus reducing the flux generated and thus reducing the torque produced. But....if it's in the acceptable range I'd say it's more likely that it's fine. The resistance will vary with temperature a fair amount so you could try measuring it when it's hot as it will probably yield a higher resistance reading then.
 

·
Registered
2003 Passat GLS 1.8T MT
Joined
·
16 Posts
Discussion Starter · #3 ·
Thanks for the informative response Iowegian. I just followed your suggestion and took the car for a spin, maybe 5 miles. Resistance is now 0.8 ohms. But I just wonder how informative this number is. I mean the difference between 0.8 and 1.2 is not that great. Regardless, it does seem to suggest the resistance is certainly on the low end of the scale. I'll take additional readings and see where it goes. Thanks again!
 

·
Registered
Joined
·
2,214 Posts
I am not sure to how to check fuel pressure regulator.... with that said you can check vacuum/hose at regulator for something ? I learned here at PW not to throw money at a problem, however when I did an extensive replace and repair on my AWM which is 20 years old... for the price I just got a new one of these, not sure if they present a catastrophic failure, but I do not like the thought of being in BFE. i.e., I carry an extra CPS in glove box. lol
 

·
Registered
2003 Passat GLS 1.8T MT
Joined
·
16 Posts
Discussion Starter · #5 ·
Thanks for the comment cchief22. Actually, I just changed out the FPR and all the vac lines are solid, so the problems lies elsewhere. I've now checked the fuel pressure and get 47/57 psi with and without vac to FPR, respectively. Although these numbers are a tad low compared to spec, I suspect they are acceptable. So if unmetered air is not the problem, and fuel delivery is not the problem, the only thing I can come up with is a faulty upstream O2 (air:fuel) sensor. I neglected to mention in my original post that the vehicle runs perfectly fine. Absolutely no performance issues whatsoever. Of course, one response to that is that the ECU is doing its job and compensating for the LEAN condition. Alternatively, there's nothing wrong with the air:fuel mixture and the upstream sensor is simply giving bad information, and causing the ECU to dump in extra fuel that's not needed.

I'd be interested to know if anyone thinks the slightly low fuel pressure values I cited could be causing a fuel delivery issue.

Thanks
 

·
Registered
Joined
·
2,214 Posts
just for "sport" pull the at pipe coming from SAIP at the combi and plug the combi at the rubber elbow. If the combi is stuck open or vacuum actuation is open do to stuck valve under intake manifold this could possibly be triggering a lean condition
 

·
Registered
Joined
·
5,544 Posts
just for "sport" pull the at pipe coming from SAIP at the combi and plug the combi at the rubber elbow. If the combi is stuck open or vacuum actuation is open do to stuck valve under intake manifold this could possibly be triggering a lean condition
Doesn't the Combi valve just admit air from the blower into the exhaust manifold- when the SAI pump is running? If the Combi was stuck open, but the pump Off, then I wouldn't expect that air would flow in, but rather exhaust flow out, of that valve. That condition is a reason, I believe, why the SAI blowers fail from exhaust-water contamination.
 

·
Registered
Joined
·
2,214 Posts
Doesn't the Combi valve just admit air from the blower into the exhaust manifold- when the SAI pump is running? If the Combi was stuck open, but the pump Off, then I wouldn't expect that air would flow in, but rather exhaust flow out, of that valve. That condition is a reason, I believe, why the SAI blowers fail from exhaust-water contamination.
you are correct, OP says he is working on a lean condition, my thinking is the O2 sensors are somehow being initially fooled and with a turbo which draws off the exhaust who knows how this all plays out......you absolutely are correct though. I am also wondering about the N-75.
 

·
Registered
2003 Passat GLS 1.8T MT
Joined
·
16 Posts
Discussion Starter · #9 ·
Thanks for the idea cchief22, and for the clarification ylwagon. I'm no expert on the components mentioned, but it sounds like a "stuck open" condition at the combi would not necessarily lead to unmetered air entering the exhaust manifold.

cchief22, you mentioned "vacuum actuation is open do to stuck valve under intake manifold". What valve are you referencing here?

Please keep in mind that the vehicle has no performance issues. It runs perfectly fine under all driving conditions, including boost.
 

·
Registered
Joined
·
2,214 Posts
yes I would rather be surfing, lol. Yes #112 ,as a side note both valves are the same but have different P/Ns as to what function they perform. When first "cold" starting your SAIP of course comes on. The combi opens via the 112 valve vacuum/line running from under intake manifold to combi. To check its operation put a vacuum gauge or maybe finger would do. Number of scenarios here; If 112 is stuck open, combi is open all the time. If 112 is stuck shut, combi never opens and cat sensor never heats up in initial start up specified time . If the diaphragm in the combi is torn or not airtight the vacuum line leaks to intake manifold thru the 112. The vacuum line from 112 to combi should be air tight with engine off. Probably grasping at straws but just trying to rule out "suspects".
 

·
Registered
2003 Passat GLS 1.8T MT
Joined
·
16 Posts
Discussion Starter · #13 ·
Thanks for the explanation...very helpful. Would you expect the condition(s) you describe to cause a performance issue?
 

·
Registered
Joined
·
2,214 Posts
Thanks for the explanation...very helpful. Would you expect the condition(s) you describe to cause a performance issue?
wish I had a definitive answer, there is a lot going on with these engines and modern engines as a whole considering all the feedback and the ECM making instant and continuous corrections. i.e. a small vacuum leak could not noticed as the ECM may correct for it. More often than not it is not the old train of thought, " if this is good, then check the next sequence ,if not, then this." My approach has been to use "logic" like Spock, but I find myself just checking one thing at a time and eliminating it.
 

·
Registered
2003 Passat GLS 1.8T MT
Joined
·
16 Posts
Discussion Starter · #15 ·
Well said. In my original post I didn't give much detail, so let me share and see what you think....

Problem stated with a P0171 flash....yet NO performance issues whatsoever under any driving conditions.

SOURCEACTION/OUTCOME
Vac LeakSmoke test (boost and engine) showed no leaks. Consistent with no leaks is the fact that fuel trim increases, not decreases, with RPM ......................................................................................................................................................
MAFCleaned with CRC
Fuel FilterNew Bosch 71028
InjectorsServiced. Cleaned with new components (gaskets, basket, pintle cap).
FPRNew Bosch 078133534C
PlugsNew NGK PFR6Q set at 0.032"
CoilsNew Bosch 06A905115D
A/F SensorPropane enrichment yields (-) spike in mA, vac leak at manifold yields (+) spike in mA. Snap throttle test reveals correct waveform (spike +mA, spike -mA, gradual return to baseline).
Fuel PumpResistance across pins 1 & 4 now reading 0.6 ohm. Outside cited tolerance of 1-3 ohms. I've been suspicious about the fuel pump from the beginning because of low resistance. However, fuel pressure appears correct (see next).
Fuel Pressure47 psi with FPR in-line, 57 psi base (FPR capped). Tested under load and stable.

I'd like to have more information on how the injectors are working, but currently don't have a licensed copy of VCDS (or similar) to scrutinize. It could be that the waveform is off, either due to the injector itself or the ECU. Also, I've closely monitored the fuel trim response to the A/F lambda sensor, and while they seem to be in general agreement, there is likely a level of detail here (either my understanding or the actual data) that may be missing. I actually bought a new Bosch 17351 lambda sensor to rule out the sensor, considering that's who's chirping. Of course, there are a host of other potential players in the lean fuel trim game. Base engine issues come to mind. But for now I'm trying to tackle the easy/most likely candidates. For what it's worth, the lean fuel trim issue I'm dealing with is not subtle. LTFT can go from 0 to +20 in a matter of 15-20 min of driving time. That seems remarkable to me, to the extent that the fix should be easy to spot. Right? Guess again!
 

·
Registered
Joined
·
5,544 Posts
Thanks for the explanation...very helpful. Would you expect the condition(s) you describe to cause a performance issue?
The scenarios that cchief22 describes wouldn't have any effect on how the engine runs, as far as I know. That's because the "secondary air" being blown into the exhaust manifold is separate from the metered induction air. During warm-up, the extra secondary air in the exhaust would obviously cause the forward O2 sensor (S1) to sense that the mixture is way too lean, but S1 would be ignored during this period. If the SAI system turned on for some reason while driving, that would cause a lean trim - but that seems unlikely to happen.
 

·
Registered
Joined
·
2,214 Posts
this is definitely a puzzler, I have a long commute so have been pondering.....two other suspects, not knowing where OP location is and mileage on car , condition of cat, and gas (winter or summer blend). I will be first to admit am still learning but am a curious sort, and have to admit I did take my Grandfathers pocket watch apart and did put it back together albeit 15 years later.
 

·
Registered
2003 Passat GLS 1.8T MT
Joined
·
16 Posts
Discussion Starter · #18 ·
Funny re your watch...I can definitely relate! I'm in NC, but the car spent its life in PA (I took ownership from my brother just last year). Mileage is 112,000. I know he kept up on service, but then again I found zip ties on vac lines under the manifold (changed out to proper clamps of course). Uncertain about the type of gas he used, but probably cheap from wholesale club pumps (e.g. Costco). Yesterday I finally got the cat readiness monitor to "OK" which has been keeping me from getting the car inspected. This happened during a hard WOT run to double-check MAF (185 g/s @ 5900 rpm). As you can see from the plots below, the relationship between the MAF, lambda O2 sensor, and STFT appears to be correct, at least to my eye (note: STFT data points of "0" indicated open-loop). If nothing else this data suggests the MAF and lambda O2 sensor are fine, so I can direct my focus elsewhere.

Interestingly, the LTFT seems to have stabilized at +10. I'll take a few more runs today and see if that's true. But I will say that I'm beginning to wonder if the problem is more a collection of "minor" issues (i.e. small leaks, sub-optimal valves, etc) that by themselves are benign. Yet together yield the lean condition.

103101
 

·
Moderator/Administrator
Joined
·
25,200 Posts
Just came to say that there is nothing wrong with zip ties on the little vacuum lines. They work well if you use a quality tie with the right amount of tension ;). I still have a couple I put on years ago.
 
  • Like
Reactions: Tomvw and cchief22
1 - 20 of 28 Posts
Top