3500 rpm stumble 240zx 280zx Maxima L motor - FIXED!

[ggzilla]

Its not the ECU

 

More likely that nothing is bad, just a loose connection.

[ggzilla]

Does it not have an under-dash fuse box?

[HRH]

It does have a fusebox underneath the dash, it's pristine being inside.  However, I think I may have the answer.  I was talking with Flatcat about it, and it occurred to me the only thing that changed before this problem arose was I put a new (rebuilt) distributor on it from a '79 280zx.  It came with a module.  The connector pins on it were a little bent, but didn't think much about it.  An ignition module getting too hot and failing would certainly cause this sort of issue.  Cools off for a second, comes back to life.  I'm going to put the old module I know to be good on here in a second and test again.  Kinda going to kick myself if that's all it was.  At least I got switches and a new coil I guess.  We shall see.

[LenRobertson]

My 510 was cutting out at speed due to a loose spade connecter at one of the matchbox blades. I can't remember what kind of dizzy you have now. You could hot wire from the coil to the dizzy and see if that helps. Fill your trunk with extra spools of wire and connectors to make jumper wires if you need em on the way to Canby. I ran my 200SX for years with the relay for the tach and gauges dead. The relay was on the passenger side under the dash, so I rigged a jumper wire I'd plug in if I wanted gauges. After a long period like that I finally wired a toggle switch into the jumper. Modern conveniences! I still don't quite understand why Karen doesn't want to go for Datsun rides.

 

Len

[HRH]

FUCK A DOUBLE DUCK!!  The module was indeed the problem.  Put the old dirty matchbox back on, better power, no issues!  Freaking sweet!  I'm soo happy now.  Going to do the rest of the vacuum lines in the morning and probably put on my new NGK wires (I'll save the old Beldens just in case) and I'm calling it good.  Then it's getting stuff ready and Tuesday morning leaving for Oregon!  Woo hoo!

 

Haha, ten steps ahead of you Len, I'm already gathering my "road box" for spare stuff and I bought two new 10' alligator clip jump leads for just such an occasion.  Need to get some more electrical tape though.  The moral of the story is new parts can be bad.  Not that I'm unhappy I wired the ignition to toggles and a push button, rather like that, plus it will solve the keys getting through out by the forces of cornering during autocross.  Always annoying taking a left hander and wing-crash! the keys fly across the cabin.  Actually, that's in the white truck, the key hole is more straight on in the Maxima, but they still tend to fall out.

[Guest Rick-rat]

Oh ,Ye of little faith...tools, meh all I took to Az. was an extra qt. of oil, gallon of water, can of fix a flat and of course duct tape

[HRH]

Oh I have faith, but this car has been heavily abused in it's lifetime.  Plus you'd probably have better luck finding 620 parts than 810 Maxima parts  in BFE if you break down. ;)  They really aren't very common.

[HRH]

After replacing all the vacuum lines and the spark plug wires, Ugly Betty runs like a swiss fucking watch!!  However, that 3500 rpm stumble on half throttle is still there a bit.  I'm almost wondering if the EGR valve is a letting in a little too much since I opened up the airbox and exhaust.  It's like its lugging the motor and it's chugging.  The solution is to not load it hard like that.  And realistically, as good as it runs, it's still a 150k motor, which maybe the valves aren't as nice as they could be.  And the EFI is pretty old so maybe that's it.  It runs pretty tits though, so I'm likely not going to screw too much more with it for a while.  It runs hard when your foot's planted, that's enough for me.

[HRH]

Double post fail!

[ggzilla]

Why wonder? Disconnect the EGR hose and plug it, test drive it, then you can tell us if the EGR was causing it.

[HRH]

Okay, back on this issue.  Having gone through almost everything, have confirmed with a vacuum gauge it is NOT related to engine vacuum.  Gauge holds steady while misses occur during mid-throttle.  Injector clips being old and missing retainers have been suggested as perhaps the harmonics or the load at the mid-throttle point tend to unseat them for a second.

 

The other thought is a problematic spark plug.  My Datsun guru had a cracked insulator on one of his plugs and had almost the same problem.  Fine at idle and WOT, but during mid up a hill, it would cut out and backfire.  I replaced the plugs tonight, could not find issue one with the old plugs short of perhaps being a little too lean for my taste in color, but still okay.  I also took the opportunity to do a compression test to further erase any wonder.

 

Also interesting is the FSM on the back has quick specs which says the L24E standard compression pressure is 171 psi.  That is for an '83, but I can't imagine them bumping up the compression for '84 only.  Short of it is, mine reads 195,190,195,198,190,196.  Roughly, the gauge isn't super particular with the increments.  Either way, quite a bit higher than the standard, and definitely not the minimum 128 psi.  In my thinking at sea level the compression ratio would be higher right?  There's denser air to compress?  As far as I know that's how it works.  Closer to sea level = more pressure.

 

Anyway, I'll take her back up the hill tomorrow on the way to work and see if the problem subsides.  If not, I'll go for the injector connectors this weekend.  Have 8 new ones with a much better clip design ready to go on.  Just a pain in the ass to cut and solder them all, but oh well!  I'm fully expecting the problem to be there tomorrow.

 

 

 

 

Oh, and pleasantly, if we take 190 and 200 to each extreme, we're still only 5% difference, so sweet!  Not bad for 150,000.  I'm wondering if the specs are wrong on the FSM or if someone has done a head gasket before and milled the head a bit.  Hard to say.

[ggzilla]

How are you going to lower the compression? A custom thickness head gasket is about $100.

[HRH]

Why would I lower it? I'm merely saying the head may have been milled before. So much the better. More compression more power!

[HRH]

FOUND IT!!!!

 

Fuel Pressure Regulator!  Ended up buying the $29 cheap ebay Honda-riffic adjustable fuel pressure regulator with gauge.  Looked functional enough.  Upon startup it spit fuel out of the four mounting screws, which were loose.  Tightened those, haven't had any problems since!  And even better, it's been cold enough in the morning, haven't had the stumble at mid throttle either!  Sooooooo fucking happy!!! :D :D :D

 

Interestingly enough, one of my accounts re-piqued my curiosity to this matter as he had just chased down the very same thing in a Ford Van that was having surge issues but kept reporting a lean condition.  Apparently the fpr was out of whack just enough to create a rich condition which in turn fooled the ecu into delivering less fuel, popping the engine code for lean condition.  Or something to that effect.  Quite bizarre.  So, sometimes when it holds vacuum, it still doesn't mean shit! ;)