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Alright, this is more of a continuation of this thread, but enough has changed that I've decided to start a new one. Some of the problems are gone, but the most annoying one is still there. Anyways, my car starts up and revs pretty well, and sounds fairly healthy. It has plenty of power, and idles around 500 once warmed up without shaking. This is a huge improvement from before, when it would barely start and idle. The car has a new distributor assembly, cap, rotor, CHTS and harness, MAF, timing belt, water pump, thermostat, and ECU. However, the biggest problem that I still have is a bit hard to describe.
At random intervals, the car will simply fail to rev. The only thing about this that seems consistent is that it doesn't usually happen when the car is idling, or I've never seen it happen at idle. It only happens when revving the car for some reason, whether standing still or driving. The throttle simply stops responding, and the revs drop to almost nothing and the engine almost nothing, actually very little sound like there is barely anything happening. Letting off the throttle causes the revs to jump and then fall back down. New vacuum hose should be in the mail, but I can't find any obvious leaks. I've tried shaking the wiring harness and it didn't cause anything to change, so I don't think it is a loose wire. I am really at a loss at this point, because when the car runs it runs very well. The fact that this can't really be replicated is really causing me trouble, and I just can't figure out what it could be.
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it? -
Stupid question, but you've gone and operated the throttle cable manually under the hood, right? It's not slipping or hung up or anything?
Has the car always been a 5-speed? What kind of ECU do you have in there? I am wondering about your throttle position sensor. -
Yes I have used the throttle cable manually, but never when the car was actually acting up. I just greased it too. Always been a 5-speed, and has an 86NA 5 Speed ECU in it. TPS is broken I think, car sometimes dies when coming to a stop from driving.
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it? -
I just caught the car doing the same thing while idling, and I was able to open the throttle body from under the hood. It didn't help, same behavior. Now, it also wants to die if I don't give it any throttle, but it still doesn't rev with the throttle open. I basically have to floor it for the engine to stay running. But what baffles me is that the car was sitting with the parking brake on and such, and it went from revving perfectly to doing this, to revving perfectly again. No apparent cause, I was watching the car the whole time. Now I do have to say that it might have something to do with vacuum, as it didn't *sound* like there was air rushing into the intake when the car was having trouble, but I can't say this for sure. Like I said I have new vacuum hoses inbound, but I don't think a single hose would be this intermittent. Perhaps the entire big vacuum canister/booster/whatever that black circular thing is called is going bad?
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it? -
That's your brake booster. If it's going bad generally you will notice that your brakes are involved in the weirdness.designate72;344989 wrote: I just caught the car doing the same thing while idling, and I was able to open the throttle body from under the hood. It didn't help, same behavior. Now, it also wants to die if I don't give it any throttle, but it still doesn't rev with the throttle open. I basically have to floor it for the engine to stay running. But what baffles me is that the car was sitting with the parking brake on and such, and it went from revving perfectly to doing this, to revving perfectly again. No apparent cause, I was watching the car the whole time. Now I do have to say that it might have something to do with vacuum, as it didn't *sound* like there was air rushing into the intake when the car was having trouble, but I can't say this for sure. Like I said I have new vacuum hoses inbound, but I don't think a single hose would be this intermittent. Perhaps the entire big vacuum canister/booster/whatever that black circular thing is called is going bad?
Check all the connections in the intake pipe, from the air filter back. WHen the car is running okay, give the pipe a good shake with your hand in several places. Tighten all the clamps on it.
It's relatively easy/definitive to isolate whether you have a vacuum hose problem. A 5-spd will run just fine with only *one* vacuum line, from the FPR to the manifold. You can unplug and cap everything else (note that "everything" in this context includes the air regulator and the idle air control valve); hook up the brake booster and the car will drive fine too. Longshot, give your EGR a wiggle too, if the valve is completely rusted out that can be a big vacuum leak too but I don't know if I have ever heard of that on a z31.
Replace your TPS, too, they're cheap. If you know something's broken and don't know if you can bypass/ignore it, replace it before trying other stuff. -
I have the ASCO block off plates for the EGR and IACV which will be installed soon. I guess I'll try swapping the brake booster from my parts car, or see if a member here has a clean one (perhaps to buy with the TPS).Hopefully the silicon hoses arrive, I figure I should replace all the hoses and then try to find any leak that might remain.
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it? -
Still, I'm wondering why it would be so intermittent. I haven't noticed that it happens every time I apply the brakes, or anything like that.
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it? -
You can easily test if it's your brake booster by disconnecting the line to the brake booster, capping that, and seeing if it helps. But it's probably not your brake booster or you'd be noticing that your brake pedal's stiff.designate72;345025 wrote: I have the ASCO block off plates for the EGR and IACV which will be installed soon. I guess I'll try swapping the brake booster from my parts car, or see if a member here has a clean one (perhaps to buy with the TPS).Hopefully the silicon hoses arrive, I figure I should replace all the hoses and then try to find any leak that might remain.
Just buy a new TPS, the manual one is like $30 on rockauto.
Test by capping the hoses at the manifold *before* replacing them. That way you will actually know what the problem was or where to look for it.
Have you actually checked any of the stuff I suggested or are you just thinking about it while sitting at the computer
Stop wondering and lift the hood.
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Well I'm not in the same location as the car anymore, so I'm speculating at the computer. However, I do want to have a plan for when I can work on the car over labor day, and I want to have all the parts I need as well.
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it? -
Sounds electrical. Check all your connectors and terminals on sensors and clean them if necessary.
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What should I use to clean them? Alternator spray from the auto parts store?
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it? -
I would get another MAF or two to test with as well, and make sure the CHTS isnt dropping out. maybe watch it's voltage at the ECU connectors?
(when my MAF went nuts, it was the third that had me running properly.)Damn dirty angels....these cars!
Current Daily Driver - 86 Turbo.
Under the cover - THE BANANA… that needs to be re-energized.
sigpic -
Pull ecu codes. It will tell you what's going on.
If none come up , then suspect vacuum leaks.http://z31performance.com/showthread…2-2-(-now-NA2T
My build thread (: -
Also, 500 rpms is way too low. Idle needs to be around 700rpms when warm.
When it fails to rev, what does the engine do? Does it shake, skip, miss, stumble, pop, smoke?
1986 300ZX Turbo…sold
1990 Skyline GT-R…new money pit
2014 Juke Nismo RS 6-speed…daily -
It is hard to tell, but the engine doesn't really seem to do anything. It doesn't shake, skip, or smoke, it is just oddly quite with occasional pops, like there just isn't any combustion. Also I have at least four mafs, of which I think three are yellow label, so I can try that when I get back to the car. CHTS is new as is subharness, and I made sure it was installed well when I did the timing belt a week or so ago.
1986 300ZX N/A 5Speed project. Needs work, but that's the point, isn't it?
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