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Voltage drop
Posted to Technical Tips Forum on 10/21/2015 33 Replies

I would like to take a moment to discuss how I was able to efficiently repair a problem vehicle with a simple volt drop test.

The vehicle in question was a 2007 Toyota Sienna. Customer stated started van in am cranked a little longer than normal. It was also beginning to run rough and surge at idle on his way here. Then the check engine light came on. The van stalled but restarted in our parking lot.

I approached the vehicle with scanner in hand. Engine scan revealed codes p0171 p0174 lean both banks. I hit the key it starts right up and idles smooth for a few seconds then the idle becomes unstable and a steady surge sets in. Pressing the throttle causes the surge to disappear and engine seems to run smooth.

So I pull some quick data and see that it is definitely running lean especially at idle. Ok lets do a visual inspection for any missing or broken vacuum or intake components. Some small vacuum lines cracked at vacuum reservoir. Repaired them, restarted still very lean at idle around 20% ltft and 2-8%stft on both banks. It is still trending lean by observing the stft.

Lets look at some more scan data. When the surge occurs I realize that the engine load pid increases dramatically. To me on a Maf car that indicates an increase in flow across the maf. I wondered why the air flow would increase intermittently. I then watched tps pids and realized the throttle was being commanded open. Did some quick graphing and saw that the idle would dip then the throttle would open then the maf and load pids would increase. So all that action was the computer trying to prevent a stall.

On to some fuel system testing. Scope pump looks weak, but not awful. Test pressure and volume found well within specs. Tested fuel quality and found no signs of water or alcohol. Now realizing fuel gauge is on full and it is pretty chilly outside I am wondering if it has a tank full of low volatility summer type fuel in it.

Next I decided to scope injectors. Pattern looked normal for the most part and as the idle dipped the pulse width would increase. So I decided to go for a quick spin up the road and it runs great under load just has a lopy idle.

So testing the fuel delivery system netted no failures. I smoke tested the engine for vacuum leaks and found none. Pinched off brake booster for no change in trims. I tried adding propane externally and watching air fuel sensors and found the did not respond to the external fuel source. I was pretty satisfied it was not a vacuum leak.

Maf tested well within spec at idle and responded quickly and accurately to increases in throttle.

At this point I have just about used up my self alloted hour for diag so I decided to walk away and fix a few other cars and come back to this one later at night when it wasn't so hectic.

Now back to it at night I started to think I either had contaminated injectors, fuel or carbon on valves. I decided to current ramp injectors to to see if I could see a definitive opening point. They actually looked normal. Other than the fact I think it looks like when the pulse width increases the current reduces. That doesn't seem right. If they are on longer then the coil has more time to charge and current should increase slightly until resistance increases, not decrease.

Is it ecm not able to hold a good ground for the injector? I don't think so because looking at the scope pattern it pulls all the way to ground and can keep it there.

Next I voltage drop tested from battery positive to injector positive. At idle it was around 800mv but just before the surge occurred it would increase to 1.3 Volts. There it is! Volt drop tested from battery positive to injector fuse shows no drop! The power goes from ignition switch to injector fuse to injector relay contacts the through the harness to the injectors. I grabbed the relay to pull it out to put my relay tester between fuse box and relay and realized relay was very warm! Installing the relay tester revealed that there was a 800mv-1.3 volt drop across relay contacts! The relay is no good. Installed new relay runs perfect. Tested total current draw across 10 amp injector fuse after repair and it was less than 5 amps so I was sure it was just a bad relay and it hadn't been ruined by excessive current draw.

I hope this long boring read can help someone understand the value of voltage drop testing and how valuable it is to have a firm understanding as to how electricity works.

Wrench on all! Thats it for now.

Robert from New York

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