tech9 Posted May 9, 2009 Share Posted May 9, 2009 I've had this jerking at low speeds for a few months now, and I can't seem to find out what it is. It happens in this sweet spot at which you have very little throttle, almost just coasting. I can get it to happen almost always now at around 25 mph or so. It also has a kind of clunking while its doing it and it jerks the whole car. It almost feels like something is loose, etc etc. I checked everything in the rear, everything visually looks good except for the mustache bushings. The driveshaft is rebuilt, fairly new fluids in diff / tranny. I ordered the master hyperflex kit and replaced the mustache bushing last night and the problem still persists. I plan on doing most of the bushings but I would love to know what this problem is, any ideas? Sounds like suspension to me, but could it be something else? Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted May 9, 2009 Share Posted May 9, 2009 How does it jerk? Like a front to back kind of jerk or does it go from side to side? Quote Link to comment Share on other sites More sharing options...
tech9 Posted May 9, 2009 Author Share Posted May 9, 2009 Front to back, almost feels like when you start a car in the wrong gear, but multiplied by 10. Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted May 9, 2009 Share Posted May 9, 2009 I really wish I could help. Personally I cant see anything in the suspension causing it, but I'm still technically a newb when it comes to cars so someone smarter should chime in. The only thing I could think of causing it would be a driveline or brake problem and I dont know what would cause the rear brakes to apply by themselves at 25mph either besides a little green gremlin with devil horns living under your floorboards and constantly pulling your e-brake cable, lol. Quote Link to comment Share on other sites More sharing options...
eec564 Posted May 9, 2009 Share Posted May 9, 2009 It's quite doubtful the problem is in the suspension. It sounds to me like your engine is misfiring. What you feel happening is the car loosing power and then regaining it again, as if you were to come off and back on the throttle rapidly. Check your plugs for fouling and proper gap. Take a good look at your cap, rotor and wires next. If you have a spare coil, try swapping that in for a bit. Does it do it specifically when the engine is hot or cold? You could have a misfire from an overly lean or rich condition. If there is a vacuum leak in your intake gasket, one cylinder could easily go lean enough to misfire under light-throttle conditions, where there is no acceleration enrichment. One cylinder out of six not firing could very well cause the car to buck. Do a search for ways of locating a vacuum leak, I have a hunch you have one; an UNLIT propane torch works nicely. Quote Link to comment Share on other sites More sharing options...
HLS30RACER Posted May 9, 2009 Share Posted May 9, 2009 The clunking makes me think the problem is likely the mount for the front of the differential. when the rubber sandwiched between the metal plates finally gives up it exhibits symptoms just like you are describing. You said everything visually looks good in the rear, did you try prying on/ jacking up the nose of the diff? it often separates where the rubber is glued to the metal and therefore looks fine without further probing. it takes considerable force to move it up and down even fully separated. Quote Link to comment Share on other sites More sharing options...
tech9 Posted May 9, 2009 Author Share Posted May 9, 2009 Not the first time someone said engine, but its not, I have checked it over and over, but I will check again just in case. It happens all the time, night day hot cold. The car runs like a top, can't feel a misfire on acceleration or idle. Quote Link to comment Share on other sites More sharing options...
eec564 Posted May 9, 2009 Share Posted May 9, 2009 You wouldn't feel it at idle or under acceleration. You run richer under those conditions and can easily mask the problem. Take a close look at your plugs to see if they all have the exact same color, indicating the mixture is even across the cylinders. You could also try enriching your entire fuel curve by loosening the spring in the AFM very slightly. Can you get it to happen in neutral? Rev the engine until it's just off the idle TPS contacts and listen/watch under the hood for an intermittent misfire. Quote Link to comment Share on other sites More sharing options...
tech9 Posted May 10, 2009 Author Share Posted May 10, 2009 Ok a few interesting things I found out today. Car drives great on the highway now from the poly mustache bushings! I guess I lied about the acceleration part, i checked everything, replaced the rotor / cap, checked plugs wires, etc. Not really any missing that I can tell at least. But it happens at more times then I expected. When i'm in 4th gear going around 30-35 and i slow down at say a stoplight, it jerks a lot with no gas pedal. Without any load on the diff, in Neutral / Clutch in, everything seems fine. So i'm guessing maybe the front diff mount, I mean what else could it be? Quote Link to comment Share on other sites More sharing options...
stravi757 Posted May 10, 2009 Share Posted May 10, 2009 im pretty sure eec564 is right about it being a vacuum leak, i have the exact same problem with my car and it turned out i had a HUGE leak on my intake mani gasket. Quote Link to comment Share on other sites More sharing options...
eec564 Posted May 10, 2009 Share Posted May 10, 2009 I speak from experience. I used to have a big vacuum leak, now I have a small one. My car can still buck, but I spend so little time at light throttle, I hardly notice. Those intake/exhaust gaskets are cheap, but a pain to swap. The biggest problem is broken bolts/studs and warped manifolds. My main problem is actually exhaust leaks. It does effect the mixture, as flow out of one or two cylinder is disrupted, causing them to fill differently. All the same in the end. Checking your plugs can usually tell you what cylinders to look near. Quote Link to comment Share on other sites More sharing options...
tech9 Posted May 10, 2009 Author Share Posted May 10, 2009 When you let off the gas on stock EFI computer isn't there no fuel being used till under so many RPM? Quote Link to comment Share on other sites More sharing options...
eec564 Posted May 10, 2009 Share Posted May 10, 2009 Yea, but that rpm is 1300 rpm. At least in theory. Old components go out of spec. You can correct most of it by playing with the AFM, and an o2 sensor cleans the mixture up a lot. Be sure to also check the internals of your distributor. If your vacuum advance gets stuck in the advanced position, you'll have ignition way too early at low rpm and the engine will fire against itself. Search for "Breaker plate". Quote Link to comment Share on other sites More sharing options...
tech9 Posted May 10, 2009 Author Share Posted May 10, 2009 I will try some more things tomorrow, thanks for all the help. I have changed to a 280zx distributor, maybe i'll swap another one in there and try it also. I can't wait to get rid of this stuff and get all my new stuff in... Quote Link to comment Share on other sites More sharing options...
eec564 Posted May 10, 2009 Share Posted May 10, 2009 The 280ZX also has a breaker plate. They're ALL broken. I replaced mine with some parts I made on a lathe. I didn't have 70$ to get a new one from nissan at the time, but I had access to a lathe. See here: http://atlanticz.ca/zclub/techtips/distributorrebuild/index.html It's easy to test on the car if you have a reliable vacuum source. Just check for smooth and even movement of the vacuum pot and internal components when applying and releasing vacuum. New stuff is always good, although old stuff isn't always bad. As long as it works properly. What new stuff do you have/are you planning? Quote Link to comment Share on other sites More sharing options...
tech9 Posted May 10, 2009 Author Share Posted May 10, 2009 EDIS, well sort of, going to run LSx coils direct from MS. Hopefully. Have pretty much new everything else also, f54, p90, hy35, MS, etc etc. Just trying to get the car in check till I get the new stuff all done! I do have a spare dissy that I can try though, at least rule that out before I tackle the intake gasket maybe! Quote Link to comment Share on other sites More sharing options...
eec564 Posted May 10, 2009 Share Posted May 10, 2009 Pull apart that spare dizzy right quick to check if the vacuum advance is still in one piece before you swap it in. Take a good look on the atlanticz page I linked above. You can also search for intake gasket leaks using an unlit propane torch. Position the torch near the suspected leak, and listen or have someone watch for the idle to increase. How steady is your idle? An idle that hunts up and down looking for a stable place is a telltale sign of a vacuum leak. Quote Link to comment Share on other sites More sharing options...
tech9 Posted May 10, 2009 Author Share Posted May 10, 2009 Well I can tell you the breaker plate is defiantly broken, since I did find ball bearings in it loose. I actually have 3, I wonder if I can convert into 1 good one maybe. Are the auto parts reman ones any good? Quote Link to comment Share on other sites More sharing options...
eec564 Posted May 10, 2009 Share Posted May 10, 2009 I seriously doubt you'll be able to make one good distributor from 30+ year old parts. ALL of the breaker plate assemblies break after 15 or so years. I've yet to ever see a good one. Even on low mileage cars the plastic breaks down. You're better off taking a distributor with very little (if any) shaft play, of the type/year range you want to use, and rebuilding it yourself. A new breaker plate assembly was around 70$ from Nissan last time I checked, and new bushings were only a couple bucks each. It's not that bad to do the rebuild, a bit easier than the AtlanticZ page makes it out to be. It's also nice to see what's stamped on your weights so you'll know exactly how much mechanical advance that distributor is supposed to supply. Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted June 5, 2009 Share Posted June 5, 2009 It's a combination of drivetrain torsional "slack" and the EFI tune. A very responsive tune can also cause this jumpiness. Mine is pretty bad ATM because I have been too busy on other things to tinker with it. The concept of "no fuel" when off the throttle is called DFCO - Decel Fuel Cutoff. I don't think that Megasquirt 2 has it; I haven't seen it. IMHO, ignore it or work around it and install the new stuff. May very well go away when you do. Quote Link to comment Share on other sites More sharing options...
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