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Unstable idle


sewebster
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So, once my truck (91) warms up, it idles at just about 750. This is the correct idle speed. Unfortunately sometimes the oil light comes on. I have a gauge and it's pretty much bottomed out. Of course, I'm not really sure how accurate this gauge is at low pressure and the light is tied into the aftermarket gauge too. The oil pressure at 3200 rpm is around 50 psi, which is just below the recommended 53-60something.

 

Anyway, sometimes the rpm drops to almost 500 and then goes back up. When stopped, even when it's at 750 it's actually really moving around between 700 and 800. It doesn't obviously miss or anything, and it sounds fine, but just changes speed slightly.

 

I decided that, as a possible short term solution I would increase the idle speed to try to insure that I have enough oil flowing around. So I followed the procedure in the manual and unplugged the AAC valve and turned the screw in the increasing (counterclockwise) direction. Nothing. Maxxed it out and no effect. So I thought that maybe my AAC valve was toast so I looked up the test procedure for that and it says that if the idle speed increases when you plug it in, it's good. This works, jumps up about 50 rpm when plugged in. I also measured the resistance and it's about 10 ohms as expected.

 

Any reasons you can think of as to why I can't adjust my idle?

 

There is an unstable idle diagnostic in the manual... but I've kinda done most of it already. I have new plugs, wires, rotor, cap, injectors cleaned and not leaking, fuel pressure checks out. Wiggling maf didn't seem to do anything. New o2 sensor too, no codes... HMM.

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Any ideas on whether the dealer would be a reasonable place to take the truck for a problem like this? Or just a waste of time/money? I have no idea how much they'd charge for basically poking around at the engine trying to figure it out...

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There is an unstable idle diagnostic in the manual... but I've kinda done most of it already.

Without just throwing $ at it, I reccommend you follow the diagnostic all the way, it sounds like you are on the right track. I would question if you have checked for vacuum leaks and/or EGR valve function.

 

B

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Without just throwing $ at it, I reccommend you follow the diagnostic all the way, it sounds like you are on the right track. I would question if you have checked for vacuum leaks and/or EGR valve function.

 

B

 

Yeah, makes sense. Do you know if the procedure of pinching the hose that leads to the PCV valve is accurate at detecting vacuum leaks? Or just finds some of them? I guess I need to do more reading on the EGR too. I know it moves by hand at least. If I pinch off the vacuum tube to it would that leave it open or closed. Obviously it isn't really necessary to eliminating EGR for diagnostics would be fine.

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To check for vacuum leaks, I always just took some carb cleaner and strategically sprayed it at the vacuum lines to find the leaking one. It could even be that it is a change in the voltage from the alternator and that is causing the lowering RPMs. Couple questions, is it auto? Do you have that little ECU under the passenger seat that has an idle adjustment on it? My 94 did, and that was the only way I could adjust my idle! I had the same problem, and once I bumped that up to be about 900 in park, and roughly 700 in drive, it stopped for the most part. Now that it has been a while since a tune-up, it slightly came back. But I would do what Precise said, do all the diagnostics, check for vacuum leaks, check for EGR clogs and proper function (which typically if it moves by hand it is functioning, but you can test it with a vacuum guage and see if that makes the diaphram move. If that doesn't do it, testing the output on the alternator couldn't hurt, as it might have gotten weak and a little extra draw may cause it to bog down...rare, but I have seen it. Let us know what you come up with!

 

<EDIT>

You may also need a new oil pressure sending unit, or it could be a bad connection between the wiring and the light...worst case scenario, you could have a plugged oil pickup tube, or even worse, a failing oil pump, but those usually just go out, not get weak.

Edited by Formerly_94PathyMan
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To check for vacuum leaks, I always just took some carb cleaner and strategically sprayed it at the vacuum lines to find the leaking one. It could even be that it is a change in the voltage from the alternator and that is causing the lowering RPMs. Couple questions, is it auto? Do you have that little ECU under the passenger seat that has an idle adjustment on it? My 94 did, and that was the only way I could adjust my idle! I had the same problem, and once I bumped that up to be about 900 in park, and roughly 700 in drive, it stopped for the most part. Now that it has been a while since a tune-up, it slightly came back. But I would do what Precise said, do all the diagnostics, check for vacuum leaks, check for EGR clogs and proper function (which typically if it moves by hand it is functioning, but you can test it with a vacuum guage and see if that makes the diaphram move. If that doesn't do it, testing the output on the alternator couldn't hurt, as it might have gotten weak and a little extra draw may cause it to bog down...rare, but I have seen it. Let us know what you come up with!

 

<EDIT>

You may also need a new oil pressure sending unit, or it could be a bad connection between the wiring and the light...worst case scenario, you could have a plugged oil pickup tube, or even worse, a failing oil pump, but those usually just go out, not get weak.

 

It's a manual transmission. Hmm, alternator might be a possibility. As for the oil, if there is a problem my guess would be the o-ring at the top of the pickup tube letting it suck in air. The o-ring I put in there when I replaced it wasn't very good. In hindsight...

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It's a manual transmission. Hmm, alternator might be a possibility. As for the oil, if there is a problem my guess would be the o-ring at the top of the pickup tube letting it suck in air. The o-ring I put in there when I replaced it wasn't very good. In hindsight...

 

Well then my guess is on that for the oil issue. Since it is a manual, I would check to see if your ECM under the passenger seat has that adjustment screw on it and try that. I always tried to put my manual cars around 800-850 just because I live in Colorado, and cold weather affects idle, and who wants to have to adjust their idle in the freggin cold! But yea, try checking the output on the alternator, just for S&G's, it never hurts to have peace of mind!

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Yeah, makes sense. Do you know if the procedure of pinching the hose that leads to the PCV valve is accurate at detecting vacuum leaks? Or just finds some of them? I guess I need to do more reading on the EGR too. I know it moves by hand at least. If I pinch off the vacuum tube to it would that leave it open or closed. Obviously it isn't really necessary to eliminating EGR for diagnostics would be fine.

Hose pinching? I never heard of that. :shrug:

I use a flammable aerosol and spray around suspected leak areas. If there is a leak, usually the idle will surge. Otherwise, pinching/moving hoses can also affect the idle if it has a leak and you disturb it. I don't know that you have this problem but I have heard of too many cases of intermittent idle not to mention it.

 

As for the EGR, the spring/diaphram should move fairly easily and it should affect RPM at some point, but I can't recall ATM so you better read up. :shrug:

 

B

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Hose pinching? I never heard of that. :shrug:

I use a flammable aerosol and spray around suspected leak areas. If there is a leak, usually the idle will surge. Otherwise, pinching/moving hoses can also affect the idle if it has a leak and you disturb it. I don't know that you have this problem but I have heard of too many cases of intermittent idle not to mention it.

 

As for the EGR, the spring/diaphram should move fairly easily and it should affect RPM at some point, but I can't recall ATM so you better read up. :shrug:

 

B

 

 

Yeah, the hose pinching thing is the 8th step in the unstable idle diagnosis. "Check for intake air leak. When pinching blow-by hose (lowering the blow-by air supply), does the engine speed rise? If yes you have an air leak. The picture shows a dude pinching the pcv hose with pliers.

 

Anyway, the oil thing is a bit of a sideline. I'm more wondering why I can't adjust the idle speed with the screw. Anyway, more searching I guess. There are a ton of diagnostics in the manual... the EF & EC section is quite good, it might just take a while, especially since it rains so much and I have no garage :thumbsdown:

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....

Anyway, sometimes the rpm drops to almost 500 and then goes back up. When stopped, even when it's at 750 it's actually really moving around between 700 and 800. It doesn't obviously miss or anything, and it sounds fine, but just changes speed slightly.

 

I decided that, as a possible short term solution I would increase the idle speed to try to insure that I have enough oil flowing around. ....

I wouldn't touch the idle adjustment..... .

 

 

 

[EDIT: The following comment is misleading..... What can I say, we all learn..... ]

 

 

 

These symptoms are common. You need to clean the airbody throttle control valve and area. It has been discussed often here and elsewhere.

Edited by westslope
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I wouldn't touch the idle adjustment..... .

 

It was already set "incorrectly" before so part of why I wanted to turn it now was to just set the screw "properly"...

 

These symptoms are common. You need to clean the airbody throttle control valve and area. It has been discussed often here and elsewhere.

 

You just mean the throttle? I don't think it's that dirty, but easy to pull the air filter and check.

 

I'm tempted to take apart the idle adjust body, but I'm loath to take off the upper plenum again (already had it off a half dozen times in the past few months).

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why would you need to remove the upper intake manifold again and again? sounds like thats your problem... as for the oil pressure problem, if your pressure is below spec at the given rpm it usually means worn bearings. it is normal for the oil light to turn on at very low rpm.

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why would you need to remove the upper intake manifold again and again? sounds like thats your problem... as for the oil pressure problem, if your pressure is below spec at the given rpm it usually means worn bearings. it is normal for the oil light to turn on at very low rpm.

 

I've had the top off for a variety of reasons, bad lifters, stuck valves, bad injectors etc. but all those problems should be solved. I shouldn't have worn bearings since I just replaced them all.

 

Anyway, I'm going to see if I can fix my o-pickup tube o-ring this weekend since I know it isn't particularly good right now. We'll see if that helps with the oil pressure, if not, I'll see where to go from there.

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Pulled the oil pan today. Took about 6 hours. In some ways it was actually easier than I expected. I didn't actually have to lift the engine to get clearance at all, so if you do it, you can try without unbolting the engine and transmission mounts. CV joint bolts require 12 mm 12-point by the way.

 

The o-ring wasn't quite as poorly sized as I had remembered though, so I'm not so sure that was the issue. I guess now I need to debate whether I should take my oil pump off to look at it again... though it seemed fine when I inspected it a couple months ago during the rebuild. Taking it off requires removing the timing belt of course, so another couple hours there. Hmm.

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How do the wires and plug to the MAF sensor look? Cleaned it(MAF Sensor) lately? I had a ton of problems out of mine for a couple of months. Dieing at a redlight if I didn't keep my foot on the gas. Then if it died I would have to pop the hood and unplug/replug the MAF harness to get it to recrank. Also you might want to check the TPS. Have you ran any seafoam thru it yet?

I wouldn't take it to Nissan unless you know the guy's there have some brains... Most likely they will just do all the same things you already did w/out finding anything but a bill for you to pay. I went down that road when my pathy was cranking forward but starting backwards...(I have video proof of the backward running engine) They told me I needed to leave it with them for a couple of weeks and that they wouldn't promise anything but $60per hr. to find the problem. They also told me it just about HAD to be the ECU and not anything on the engine... The guy on the phone didn't think our engines could start backwards and basically called me a dumbass. Turned out to be a faulty distributor.(discovered using a timing light. lol)

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I cleaned the MAF recently, wasn't very dirty. Wiggling the wires didn't do anything.

 

I've sortof stopped caring so much about the slightly unstable idle. Before I thought that was why I had the oil light come on (low rpm) but now I just want to find the oil problem.

 

Anyway, to that end, I took off the oil pump today. I had tried blowing in the bottom (where the pick up tube attaches) and heard some air seemingly coming out nearby, which seemed bad. Once I got it off and blew through it again, I could tell that the air was leaking out between the gear and the cover plate where the crankshaft goes through. This is likely my problem. Cover plate must have been worn down over time by the gear and now it doesn't seal. So, need a new pump.

 

Tried taking the CVs out the inside while I have the front diff off... haven't gotten it yet and not sure if it can be done. There isn't much room between the shock, lower a-arm and sway bar. Maybe if I take the shock and sway bar off it will work, but the problem is that you can only jack up the lower a-arm so far before the tension in the t-bar starts lifting the truck.

 

Now to see how fast I can get an oil pump. Unfortunately the truck is in a friends garage and I don't want to overstay my welcome too much!

Edited by sewebster
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Replaced the oil pump. Pressure seems to be good now! Hopefully it stays that way. I do recall seeing higher pressures right after oil changes before (fresh filter?). Anyway, fingers crossed...

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Take apart your old pump and post some pictures of the wear. Might save someone else from using a pump that too far gone.

James

 

I actually tried to take it apart on the weekend, but the screws on the cover plate (#3 phillips) were really tight. Not sure why since I just had it apart a couple months ago.

 

Anyway, I measured all the clearances per the FSM when I had it apart, and it passed. However, I think the procedure is flawed. They have you measure stuff, including the clearance between the gears and a reference surface, such as an engineer's square. They don't have you measure the clearance between the gears and the cover plate (would be hard), which is what actually matters. My cover plate showed some wear, but I didn't think it was too bad. When I took the old pump out I covered the outlet (and the small hole on the bottom) and blew into the inlet. A lot of air leaked out between the gear and cover plate in the centre (where the crank goes). The new pump leaked much less there (still a bit since there is no real seal).

 

Anyway, if you have your pump apart I'd be carefully evaluating the cover plate and also doing the "blow test."

 

Interestingly, the new pump I bought from RockAuto (Sealed Power brand from Federal Mogul) is basically identical to my old one, except where the nissan logo was there are some grinding marks like someone removed them. So I guess it is from the same molds. Dealer pumps aren't super expensive, but it was going to be a couple weeks to get one.

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Interestingly, the new pump I bought from RockAuto (Sealed Power brand from Federal Mogul) is basically identical to my old one, except where the nissan logo was there are some grinding marks like someone removed them. So I guess it is from the same molds. Dealer pumps aren't super expensive, but it was going to be a couple weeks to get one.

 

Typical...I saw it done before.

 

40191_10150222923670231_613280230_13920539_932931_n.jpg

 

45674_10150222923695231_613280230_13920541_6741457_n.jpg

Edited by Tungsten
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  • 5 months later...

So, as a final follow up on this issue. It turns out I had two problems. One was my oil pump was toast. The other, which I just finally actually figured out was correctly guessed by B in the first response. The EGR was sticking occasionally causing the idle to be terrible when it got stuck open. The reason it was sticking was that the "vent" line for the EGR control vacuum circuit was plugged into the evap canister valve. It is supposed to be plugged into the air hose between the filter and throttle. Oops... though, I'm fairly confident it was actually hooked up that way when I bought the truck.

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