78 280Z Running Lean

[78-280Z]

Hi all,

 

My car was stuttery when I bought it (2 weeks ago) and have not driven it much since. I have been trying to tune it up. Yesterday, I put in new plugs (the ones the FSM calls for), distributor cap, and rotor as well as a new fuel filter (the one in the engine bay).. All from MSA.

 

After that, I used my timing light to adjust my timing. I verified TDC with the rotor pointed at spark plug 1, cylinder 1 was at TDC and mechanical timing was at 0. I then disconnected the vacuum advance at the distributor and capped it. I checked the timing and found it to be about ~15 advanced. FSM calls for 10 so I retarded it to 10. It idles happily at 10*. However, when I try to rev it, it's sometimes good, and other times it backfires in the intake (frontfires?). I had driven it around a bit after this and when I came back from my short drive (it front-fired a fair amount) I pulled a couple of plugs and they were pretty white. The front-fire in combination with the white plugs tells me that I am running lean. When I pulled the original plugs, they looked normal (light brown)

 

I am trying to figure out why it is running lean. I tested the AFM and these were my results

 

Resistance between 6 and 8 = 184ohms, normal

Resistance between 9 and 8 = 103ohms, normal

Continuity check of 6, 7, 8, and 9 are all 0, normal.

 

I will run 12V to the AFM when I get home to check the potentiometer readings. 

 

There is some gray silicone around the plastic cover on my AFM - does this mean someone has messed with it before or is that how it was sealed from the factory? I have yet to open it. If this means it was altered, I may open it and try and set it back to original specs using this link: http://www.atlanticz.ca/zclub/techtips/afm/ IF I can figure out what the OEM specs were. I do not want to adjust the AFM to mask a problem. I do not want to alter it any other way since you're not supposed to. I'd just want it back to stock specifications. 

 

Resistance from 27 and 8 (air temp sensor) is inconsisten(ish). I will only get a reading when I tap the my multimeter probes to them and it will give me a reading for a quick second then disappear. I can't get it to hold a reading. When I see the blip of a reading it is between 1.5k ohms and 1.8k. It's probably around 75 degrees in here and 68 degrees would be a resistance of 2.25k - 2.75k. So if those readings are accurate, it may be a little off. What concerns me is it not holding a resistance. 

 

Could it be a bad temp sensor? Would it cause my car to run lean? I am not sure what the temp sensor defaults too when it fails. If it defaults to thinking the temperature is really hot, it could not be providing enough fuel. I know the ECU only adds extra fuel when the incoming air is below 70* F. 

 

I do not have a vacuum gage or a fuel pressure gage so I cannot verify those two things at the moment. Thoughts? What is the default mode of the AFM if I disconnect it? Would it run "rich"? 

Edited May 27, 2016 by 78-280Z

[NewZed]

This tip will probably make the engine run right, although the original problem might remain - http://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

 

Fuel pressure is critical.  It needs to be known.  A weak pump, or a clogged filter or tank can cause your problem.  Vacuum leaks can also cause a lean condition.  The intake system, including the crankcase (PCV) needs to be sealed from outside air.  All air must pass through the AFM.

 

I spent a lot of time tuning my car, then found that several of my injectors were clogged.  The car sat for quite a while before I got it.   Your AFM numbers look great.  Don't mess with the AFM spring unless you're positive it's already been messed with.  Numerous people have got lost inthe AFM internals and ended up buying new ones because they didn't know how to get back to where they started.  Mark your starting point if you get in there.   There should be a blob of glue on the set screws for the cog, use a bright light and magnifying glass to see if they've been broken.  The side cover just pries straight off, no clips holding it in place.

 

The problem is very very common.

[78-280Z]

This tip will probably make the engine run right, although the original problem might remain - http://www.atlanticz...rpot/index.html

I will pick up a potentiometer on my way home today just in case I can't figure out the issue. My plan is to adjust valve lash too. The car supposedly sat for years before the PO bought it (he only had it for 6 months or so) and don't know its history before that. Who knows if it has ever been re-adjusted. Could bad valve lash cause this issue? A valve could not be closing all the way or staying open too long?

 

Fuel pressure is critical.  It needs to be known.  A weak pump, or a clogged filter or tank can cause your problem.  Vacuum leaks can also cause a lean condition.  The intake system, including the crankcase (PCV) needs to be sealed from outside air.  All air must pass through the AFM.

I am renting one on the way home today to verify this before I try the potentiometer too. From what I remember, it should be around 35-38PSI most of the time? I sprayed over every line I could find with carb cleaner and my idle did not change so I do not think I have any notable leaks unless there is one hidden somewhere that I can't find. I replaced all but 1 or 2 vacuum lines on the engine a couple of days ago to eliminate any potential vacuum leaks. I also cleaned out the PCV valve (it's not sticky - I blew in it and made sure it only flowed in one direction). There is an inline fuel filter near the pump, correct? I have not checked it yet. Any info on removing it/cleaning it?

 

I spent a lot of time tuning my car, then found that several of my injectors were clogged.  The car sat for quite a while before I got it.   Your AFM numbers look great.  Don't mess with the AFM spring unless you're positive it's already been messed with.  Numerous people have got lost inthe AFM internals and ended up buying new ones because they didn't know how to get back to where they started.  Mark your starting point if you get in there.   There should be a blob of glue on the set screws for the cog, use a bright light and magnifying glass to see if they've been broken.  The side cover just pries straight off, no clips holding it in place.

I don't plan on messing with the AFM. I've done enough research to know I should start elsewhere. All of the injectors were cleaned out before I got them. I have receipt from the PO saying he had a shop do it 6 months ago but I supposed it wouldn't hurt to do it again myself. 

 

The problem is very very common.

I found many similar posts to mine but never did anyone post back with a solution if they ever figured it out!

Edited May 27, 2016 by 78-280Z

[78-280Z]

Fuel pressure is right around 32psi. The FSM calls for 36... Is 32 OK? Or do I need to replace my FPR? My fuel filter in the engine bay is new and it looks like my fuel pump is not original and does not have a little fuel filter on it like the OEM one. Even after I shut the car off it's staying up there, telling me it's holding the pressure. 

Edited May 27, 2016 by 78-280Z

[NewZed]

Looks like you did not disconnect the vacuum hose from the FPR before measuring.  32 is close for idle but a bit high, showing low intake vacuum.  Intake vacuum pulls fuel pressure down.  But there are other things that can cause low intake vacuum.  Valve lash, for one.

 

You can either restart the engine with with the hose disconnected from the FPR, or just remove the small yellow wire from the starter solenoid and turn the key to Start.  Fuel pressure should be 36-37 psi, with no intake vacuum correction.

 

For whatever reason,many of the original AFM's, even though they test out as good, run lean and need the "fuel tweak".  That's why there's a complete write-up on it.  Could be that Nissan tuned them to the edge for emissions and economy back in the 70's, and now that we have 10% ethanol everywhere and no-lead and other formulation changes, the tune is too lean today.  If you get good fuel pressure, go to the fuel tweak.  You'll be able to drive it while you're fixing other small problems.

[78-280Z]

Zed, you're right. I did not disconnect the fpr. I can test again tomorrow but read on for my discovery today...

 

I put the old plugs back in (for the heck of it) and tweaked the timing a bit. I then noticed it was a tad better and at this point i was down to maybe a couple of gallons of gas.

 

I drove to get gas and it drove like crap on the way there. When I removed the cap, it hissed a lot. This told me there was a lot of vacuum in the tank. These cars don't have vented caps, where/how are they vented?

 

I filled it up with 93 and it drove AMAZING on my way home. No stutter, full power. It has literally never felt this good. Do you think there was so much vacuum in the tank that the fuel pump was really struggling to get the fuel to the rail causing my lean condition on a tank low on fuel?

 

PS - when i hooked up the fuel pressure gauge, I got no pressure with the key to the on position. Only once I started the car running did I get full pressure. This also makes me think fuel was struggling to get up the engine.

[NewZed]

Actually, the cap is vented.  A little valve in the center, hard to see, to let air in.  And a vent line up to the charcoal canister to let air out.  If your cap vent was clogged it might cause your tank to collapse but both inlet and return lines in the tank see the same vacuum or pressure, so fuel pressure at the injectors would still be maintained.  Could be that you just had a bad tank of gas.  Or something is floating around in the tank and it clogged your outlet port.  Filling the tank dislodged it.  It might come back.  Good luck.

 

The pump doesn't run at key On-engine not running, only Start and engine running.  The Service Manual is full of good descriptions. 

[78-280Z]

Thanks for all your help and knowledge Zed. Now to deal with my seized fan clutch...

[78-280Z]

Actually, the cap is vented.  A little valve in the center, hard to see, to let air in.  And a vent line up to the charcoal canister to let air out.  If your cap vent was clogged it might cause your tank to collapse but both inlet and return lines in the tank see the same vacuum or pressure, so fuel pressure at the injectors would still be maintained.  Could be that you just had a bad tank of gas.  Or something is floating around in the tank and it clogged your outlet port.  Filling the tank dislodged it.  It might come back.  Good luck.

 

The pump doesn't run at key On-engine not running, only Start and engine running.  The Service Manual is full of good descriptions. 

 

Hi Zed,

 

As usual, you were right. I fixed a number of other issues that I had and it ran great for a day. Then yesterday when I was a little under half a tank, it started with this whole lean + backfiring in the intake issue. I went and got gas (topped it off, added like 9.5 gallons) and now she runs like a dream again. I do believe now the only thing that could be causing this, is something blocking the fuel outlet port. I assume the only way to deal with this is to drop the tank and clean it out. Any other "less time consuming" potential issues it could be? If not, I will probably get to the tank in the next couple of weeks.

 

Fuel hose 3 here would have to be blocked?

 

It does have a new fuel filter in the engine bay.

Edited June 28, 2016 by 78-280Z

[NewZed]

Here's what you'd be dealing with - http://atlanticz.ca/zclub/techtips/fuel/gastank/index.htm

 

Some people have proposed that the pickup tube inside the tank can get a pin hole and suck air.  But it doesn't seem to rise high enough to start happening at 1/2 tank.  The hole would have to be above the fuel level.

 

At least you have a fix, to keep you going.  If the obstruction(s) is(are) floating maybe you can siphon them out from the filler tube.  Or use the drain plug and drain all of the fuel.  See what comes out.  That's probably the quickest way to assess what's in there.  Use the fuel pump to pump it as low as possible and the drain plug to finish.

[78-280Z]

Zed, thanks for your insight. I really do appreciate having you to bounce ideas off of (moreso to correct me).. I am going to drive it around for a couple of more days to use up some of the fuel and make sure that this gas "fill-up" was indeed the "fix". This will give me assurance it's worth digging into the fuel tank. I think I will do as you said and drain out the tank and see what I find. I think I may just drain it instead of using the fuel pump so that way if I do have gunk in there, I avoid pushing anything more than I need to through any lines/pump unnecessarily.