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Weak Idle EA82 SPFI / Voltage Drop Test


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13 replies to this topic

#1 MorganM

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Posted 21 June 2005 - 10:19 AM

I'm tryign to track down the source of my 88 GL's weak idle and missfire. Pretty sure it's an electrical issue of some sort. I've checked every mechanical problem I can think of.

I read this page here http://users.cnnw.ne...oltdroptest.htm and thought I understood how to do it. I tried doing it right at the battery on my main battery cables and came up with -0.00 vdc at all resistance points. So that's a good thing right; no voltage drops found? One thing they dont explain is what to set your digital multimeter too; I set mine to the 20 VDC setting.

Then I clicked on the real life example of checking the head lamp circut. Big problem; this little section was obviously NOT designed for Subaru headlights! There are 3 connectors to a single headlight bulb on my Subaru; not two like in teh example. I ended up burning out the fuses for my headlights! :( Basicly what I want to do is voltage drop tests on all critical sensors BUT I don't wanna fry anything like I did on my headlight fuses!

I think it's a problem with my IAC or TPS but I'm not sure. The idle is just very weak, there's a bad missfire at idle, it gets WAY worse the more draw I put on the electrical system... say I kick on a turn signal, headlights, or fan and I usually stall the engine at idle!

#2 NorthWet

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Posted 21 June 2005 - 10:51 AM

After skimming through that article, especially the "headlight" section, it seems to me that the person who wrote it either doesn't know what they are talking about or doesn't know how to explain things.

"Voltage drop testing" is just indirectly measuring connection resistance. Sometimes with high power connections, a connection with marginal resistance will completely fail when required to pass large amounts of current; but it is generally better to directly measure the resistance rather than infer it from the voltage drop across it. That being said...

Under what conditions did you measure voltage drop at the battery terminals? If it was under zero-load then the zero drop would be expected. It should be tested under as much load as practical. If you have problems with lights, heater-blower, and wiper motor running then this is the condition that you should test during.

I am not sure how your fuses could have blown while testing the headlights, unless a DVM probe touched both terminals at once. The 3 connectors on your headlights are for a common ground, power to lowbeam and power to highbeam. With the lights off connector off and your meter set for resistance (ohms), find which terminal has zero resistance (vs infinite resistance) and this will be your ground. With meter turned to volts and the lowbeams on, look for near battery voltage and that will be your lowbeam terminal, with the other being the highbeam. (Or you can check out a wiring diagram for this info.) Then just carefully measure from positive battery to each powered terminal, and from the ground terminal to the battery ground.

#3 MorganM

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Posted 21 June 2005 - 11:14 AM

OKay so I should do resistance testing first and if it checks out then do voltage drop testing?

Tested battery cables at a pretty minimal load. Mostly did it just to test out what I had read in that webpage. I don't really suspect any of my main battery cables ( + or - ) as I've repalced them both recently with good, clean contacts.

What you suspected is exactly how I burn out those fuses :) Again I didnt really suspect anything wrong with the headlights I just wannted to apply what I had read to a non-critical circut. So to avoid disaster when testing sensors I just need to NOT suppry voltage to the ground terminal; check?

Can you tell I'm a super newb with this here fancy wal-mart digital multimeter? :lol: I've done resistance testing before on my EJ22 when I was having problems with it and found a bad fuel injector. However I dont have OEM spec values for my EA82 to check against :( for things like the TPS, IAC, MAF, coolant temp sensor, etc etc etc.

I just want it to not stall on me :( I must find the culprit! :grin:

#4 NorthWet

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Posted 21 June 2005 - 11:30 AM

If you want to test the sensors, you really need to either a) measure the resistance directly, or B) measure the voltage at the sensor under operating conditions (this is NOT drop, just the voltage). These require knowing what the readings should be to be of any use. Without knowing what they should be, about all that you can really do is check for open and short circuits.

Regarding whether you test for resistance or voltage drop on power connections, either can work IF you understand what you are doing. Voltage drop is less useful for sensors then for power connections.

So, to your original problem of engine problems. Is your idle excessively low, so that turning on an electrical load (and the resultant engine load from the alternator) causes the RPM to drop further? Or is is more likely that a drop in system voltage due to the added electrical load is causing the rough running engine? Does this problem only occur at idle? If it happens above idle, I would think it was the "drop in system voltage" scenario, in which case checking the sensors probably won't do you much good.

#5 Cougar

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Posted 21 June 2005 - 12:06 PM

As you may already know MorganM, whenever measuring resistance you need to remove power to the circuit you are working on. When measuring resistance you also should isolate the device you are measuring by removing at least one of the leads from the circuit so the reading will not be influenced by other devices in the circuit.

You should be able to check DC voltage to any of the sensors or systems on the car with no problem. You just need to be sure you don't short the power lead to ground by slipping the probe to ground or touching the ground probe at the same time.

Another thing you can check with your meter is noise getting into places it shouldn't. By setting the meter to measure AC volts and measuring the power to the device you are looking at you should see close to zero volts. This test won't work with RF frequencies due to the response of the meter but can help in most cases.

Keep getting familiar with the meter and you will soon be grabbing it for all your electrical problems. It is a great tool.

#6 MorganM

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Posted 21 June 2005 - 12:23 PM

Yeah I'd have to run to the stealership or something to get voltage and resistance specifications to check against.

It idles at 800 RPM (+/- 100 RPM) like it should. Untill it missfires of course then it drops and comes back up. Miss fire can be as bad as two cylinders in a row. Sounds fine above idle; sometimes cuts out or bogs down when its cold but at full op temp it seems to run fine above idle.

So at idle I turn on my headlights and the idle drops say another 100 RPM. Now turn on the fan with my switch in cabin and it gets worse. Turn on a turn signal and every time the signal flahses RPMs drop more... in perfect time with the turn signal... hell I can even hear my fuel pump osilate speed in perfect time with the turn signal.

It has been suggested to me that my idle is just too low. Well I can't really turn up the idle on SPFI. I can but the computer will eventually compenstate for my adjustments at the throttle stop screw. I belive turning up the idle would only mask the problem; not solve it.

Yes spark plugs, plug wires, cap and rotor are new or in good shape and my fuel filter is replaced in the last 20,000 miles :)

#7 Bill90Loyale

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Posted 21 June 2005 - 12:35 PM

[quote name='MorganM']Can you tell I'm a super newb with this here fancy wal-mart digital multimeter? I've done resistance testing before on my EJ22 when I was having problems with it and found a bad fuel injector. However I dont have OEM spec values for my EA82 to check against for things like the TPS, IAC, MAF, coolant temp sensor, etc etc etc. /QUOTE]
Two thoughts:
1. Morgan, I've got the FSM electrical book for a 90 loyale. The testing specs are probably similar. In addition, be advised that Haynes (if you have one) also contains some of these specs. Let me know if you need help with specs.
2. This is another great example of why a newbie multimeter owner like me would benefit from a nice, elementary electrical device/troubleshooting tutoorial in the Repair and Mod section. I would wager a bet that Northwet would do a superb job at a project like this. Pat?

#8 NorthWet

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Posted 21 June 2005 - 12:36 PM

So, it sounds like your problem is only an issue at idle. 800RPM should be plenty for a manual. So, I have two thoughts, neither of them new to you...

First, there is a general electrical problem. What does your voltage gauge needle do when you start loading the electrical system? What is(are) the reading(s)? What does the needle do when the turn signal blinks? Just for kicks, have you tried attaching a battery charger to your battery while the car is running to see if stabilizing the voltage has any effect? (Only do this at a low amp setting, like 2-10 amps, and do not remove the battery cables from the battery or you risk electronic component damage.)

The other thought involves the IAC and TPS. IIRC, the TPS has a specific value when at idle; might be that a specific wire has a signal on it saying that the idle contacts are "made". The IAC may not be responding to ECU commands; if you have a fast cold idle you probably can eliminate this psossibility.

#9 MorganM

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Posted 21 June 2005 - 12:42 PM

Volts meter in the dash sits a hair above 12 volts at idle and goes upto about 14volts above idle. It to bounces in perfect time with the turn signal.... practicly everything eletrcial in the vehicle does!

We might be onto something now with the TPS. The problem has just gotten worse the last time I was out. The cold idle no longer works... I start it up for the first time that day and it just goes down to idle now. Used to shoot upto about 2000 RPM and hold till it warmed up. Now it just sits at idle or sometimes it will rev up then come back down. The idle has got worse now also; darn near have to keep my foot in it at all times to keep it running. That's why I really started digging around for a solution. It was tolerable before but now it's worse :(

#10 NorthWet

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Posted 21 June 2005 - 12:51 PM

First, easy thing to do if you haven't done so already, is to disconnect the IAC connector, check out the terminals for obvious corrosion, and then reconnect the conector.

You should still be thinking TPS also, as if the ECU is not getting the IDLE contact signal, I doubt that it would tell the IAC to fast-idle.

It still sounds like you may have a battery/cable/alternator issue, as I wouldn't expect the turn signal and such to greatly affect the voltage gauge needle; though it could be in combination with an IAC/TPS problem.

#11 MorganM

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Posted 21 June 2005 - 12:59 PM

I'll tripple check IAC plug and wires. I've checked them before but I'm kinda blind (hence the grounding out of the headlight bulbs! LOL)

I have a spare TPS to swap on. However they are adjusted properly to the throttle plate; check? How can I swap one on properly? The one on my Legacy had mark of wihte paint from the factory that they used to line it up. Should I just mimic that?

#12 NorthWet

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Posted 21 June 2005 - 01:15 PM

For the TPS stuff, try this link. It may or may not help as it was originally for an MPFI problem:
http://www.ultimates...position sensor

#13 Cougar

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Posted 21 June 2005 - 05:51 PM

From reading the new posts I also think that checking the battery condition would be good to do. The alternator will put more load on the engine as more power is needed from it.

#14 MorganM

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Posted 23 June 2005 - 10:30 AM

TPS checks out fine. Resistance test results all within spec. The search contintues :confused:




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