Guest 2slo4u Posted August 12, 2004 Share Posted August 12, 2004 I've heard that when you take the oilcap off, the engine should idle up, well mine idles down to the point that it almost dies. I could not find vacumm leaks using propane but when I put propane directly into the oil fill hole, the engine idled up and little and smoothed out! What the heck? The car idles really low (about 700-750 rpm) even after idling it up with the adjustment screw. Do these things mean it's running lean? It does not detonate but at idle up to about 1800 rpm when accelerating, there is hesitation/miss. -Fuel pressure always reads normal -Brand new turbo injectors -new spark plugs, wires, cap and rotor -have swapped ignition coil with two others, no change -egr and aac are "capped off" Quote Link to comment Share on other sites More sharing options...
Mike kZ Posted August 12, 2004 Share Posted August 12, 2004 That's weird, You have a vacuum leak at the head. The only thing I can think of is one or more valves are sticking open. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted August 12, 2004 Share Posted August 12, 2004 I believe this is normal. My Z did it, and my SX does it. I think this is due to the large vacuum leak exposed with the cap taken off. The air drawn into the manifold from the valve cover (through the large hose between the valve cover and the intake manifold) bypasses the AFM, thus it is not metered. Anybody else?? Quote Link to comment Share on other sites More sharing options...
Ivan280zt Posted August 12, 2004 Share Posted August 12, 2004 Sorry guys that i cant help but i do have one question. I belive that i have an vacum leak but i am having hell of the time finding it so how does it work with propane? I have never heard of using propane to find vacum leaks can you please explain how you do that? Thanks, Ivan Quote Link to comment Share on other sites More sharing options...
Guest 2slo4u Posted August 12, 2004 Share Posted August 12, 2004 I read on a few hybridz threads that putting propane (I used a torch with a long vacumm tube attached) would make the engine idle up when the leak sucked the propane in. I guess I don't have any external leaks because I could not get any reaction. The reason I'm looking for leaks is because my idle sounds "like a cam" according to my neighbor. It misses and you can hear it kinda like "bump.........bump,bump................bump..bump...)very erratic miss. If you're sitting in it at a light for example, the car shakes every time it misses. Almost seems like turning the key off and back on really fast. Above 2000rpm their is no miss, acceleration sounds smooth. When I let off the gas after accelerating though, you can hear popping inside of the exhaust like it's backfiring. It's not loud, but I can still hear it. 81zxturbo 95slobaru Quote Link to comment Share on other sites More sharing options...
johnc Posted August 12, 2004 Share Posted August 12, 2004 I've yet to actually see any L6 powered vehicle change idle speed when the oil cap is removed. The crankcase is vented to atmosphere via the air intake box or air cleaner assembly and a PCV controls crankcase ventillation into the intake manifold. My first guess about the problem posted is that the PCV valve is stuck, open. Quote Link to comment Share on other sites More sharing options...
Guest 2slo4u Posted August 12, 2004 Share Posted August 12, 2004 Thanks John for the input. I actually changed the pcv yesterday hoping it was bad. I also sealed the metal tube connecting the pcv hose to the crankcase because it was loose. I did not seal the tube closed, I removed and cleaned the metal tube and used sealant on the end when putting it back into the crankcase. I'm sure I cured a small vacumm leak by doing that but it runs no better. I feel like pulling my hair out, just can't understand what's going on with this engine. I'm going outside right now to put the previous stock cam and rockers on and see what happens. When the car cools down I'm going to check all of the intake nuts and bolts to make sure none have gotten loose since replacing the head a while back. 81zxturbo 95slobaru Quote Link to comment Share on other sites More sharing options...
DatsunBoy77 Posted August 13, 2004 Share Posted August 13, 2004 do you have a vacuum/boost gauge. If i remember correct at idle the vacuum should be near -20hg(whaterever it is called) give or take a little. I would think that would help give you some kind of idea as if you have a vacuum leak. Ryan Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted August 13, 2004 Share Posted August 13, 2004 Whenever I've had a fuel injected Z with the PCV system fully intact, opening the oil cap or removing the dipstick with the engine running has always caused a change in idle. With the PCV system intact you are creating a vacuum leak by opening the oil cap. PCV routes into the intake manifold, past the AFM and throttle valve, of course if you open the oil cap you're going to be letting unmetered air get sucked in thru the valve cover and thru the PCV valve. Removing the dipstick creates only a small leak, and hence, idle usually goes up and gets a tad rougher. Opening the oil cap creates a HUGE leak, and on every EFI Z I've owned has caused idle to drop signficantly or even stall the engine. I'd say this is normal. Quote Link to comment Share on other sites More sharing options...
DrSideways Posted August 13, 2004 Share Posted August 13, 2004 If this is a Fuel Injected Z then the idle does go off when the cap is removed. No extra charge......... Alan Quote Link to comment Share on other sites More sharing options...
johnc Posted August 13, 2004 Share Posted August 13, 2004 Whenever I've had a fuel injected Z with the PCV system fully intact, opening the oil cap or removing the dipstick with the engine running has always caused a change in idle. And my 1977 Datsun 810 with a L24E does not exhibit this behavior. But, reading the FSM again, a vacum should be created in the crankcase at idle by a properly operating PCV valve, so that valve is open at idle. If some other aspect of idle (mixture, speed, timing) is off then I can see how pulling the oil cap will cause the problems described in the first post. Quote Link to comment Share on other sites More sharing options...
Pyro Posted August 13, 2004 Share Posted August 13, 2004 My 78 dies when I pull off the oil cap. I guess that big hose going from the valve cover to the intake creates a huge vacuum leak and shuts down the engine. I guess this would also depend on the idle A/F ratio. My car must be running lean at idle. I'm sure that is normal. Especially after viewing the "Datsun Efi Therapy Video". The video saids, if the car dies or almost dies then that means no other vacuum leaks are in the system. The Video also said that the rear main seal can cause a vacuum leak! Datsuns are crasy! Try doing a compression test, check for broken valve springs or worn cam lobes. Quote Link to comment Share on other sites More sharing options...
Guest 2slo4u Posted August 14, 2004 Share Posted August 14, 2004 Yeah that would make sense if the vacumm leak pushed the mixture past the threshold of lean to too lean to stay running. And man, that pesky rear main seal is not due to be changed until I get a bw t-5 to change mine from auto to standard. I changed the front main seal when I pulled the front cover for timing chain work. Well, until I get an a/f ration gauge, I guess I'll have to be patient. Thanks for the excellent responses everyone. 81zxturbo 95slobaru Quote Link to comment Share on other sites More sharing options...
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