Guest butlersZ Posted August 9, 2003 Share Posted August 9, 2003 I hope I didn't put this in the wrong forum but can someone help me with my damn dieseling problem. I have a 74 260 with and L28 F54 block and an E88 head with dual carbs and a 270 cam. timing is set at 8n deg. but when I turn the car off it still diesels and runs for 10-45 seconds after, even when the battery is disconnected! I have flat tops and think my comp ratio is around 10.5 or 11.0 :1 , could the high comp be the cause? It's not overheating or anything. I have had the timing set as far as 36 deg. advanced and it still diesels, any suggestions???? Quote Link to comment Share on other sites More sharing options...
Mudge Posted August 9, 2003 Share Posted August 9, 2003 Crap on the exaust valves (hot), bad plugs, or leaky carbs - sometimes these cars dont shut down if the alternator is not wired properly. Quote Link to comment Share on other sites More sharing options...
Guest Zachb55 Posted August 9, 2003 Share Posted August 9, 2003 high compression isnt the DIRECT cause. another thing that will cause dieseling is a fuel with too low an octane level. i could go really in depth with this but simply put its like this; diesel fuel has a really low octane, the lower the octane the faster the fuel will burn. if you were to lay a strip of 87 octane fuel next to a strip of 92 octane fuel and lit them at the same time, the 87 would burn down the line a lot quicker. SO, you might try using a higher octane fuel to solve this problem, this would also be a good cure if you had some preignition issues... well you might try that as a simple solution, but not a cheap one.. -Zach Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 9, 2003 Share Posted August 9, 2003 Check your mixture. Some people think Z's with SU's tend to diesel when they are rich. I always found that mine dieseled when it was lean. Lean mixture gets things hot in the combustion chamber, like the edge of the valves, those carbon deposits, etc. I'd turn the mixture screws out 1/4 to 1/2 turn and see what that does. Jon Quote Link to comment Share on other sites More sharing options...
DavD3 Posted August 9, 2003 Share Posted August 9, 2003 Somethimes excessive heat could be the reason for dieseling.I had this problem, and dicovered it excidently: with hood open there was no dieseling whatsoever, with closed hood, car was running and i had to shot it of by put it in 1st gear and release clutch. Swithching to 280z hood with vents helped.If you have headers maybe this is the reason.Just my .02 Quote Link to comment Share on other sites More sharing options...
Guest rick458 Posted August 9, 2003 Share Posted August 9, 2003 Make sure your Idle speed is correct a high Idle speed will almost always lead to deiseling Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted August 9, 2003 Share Posted August 9, 2003 being too lean will cause a mixture that will ignite very easily and can lead to over-run being too rich shouldnt be the problem pull your spark plugs and check to see if they show signs of predetination or burning too lean (i believe this is a whiteish colored electrode that looks almost like calcium deposits like what you'd see around a leaky faucet) I can double-check this if you need. I would be hesitant to up the octane until you confirm that you are deposit-free. The reason I say this is that octane causes a resistance to burning, if you have a condition causing deposits in your engine then higher octane is likely to lead to incomplete combustion and even more deposits. I would run some of chevron's techron additive through and see if that helps and then maybe run a little higher octane as that CR might be too high for 87 octane. Another important note is to notice if your engine is cranking in the normal rotation when you turn it off (actually called over-run) or if it kicks backwards and runs backwards (what used to be referred to as dieseling). Of course, the terms are just as they have been explained to me, someone else feel free to chime in and back me up or correct me .. and runs for 10-45 seconds after, even when the battery is disconnected! Oh ya, a note on this, if the problem is simply that your alt is still firing then disconnecting the battery wouldnt help at all. This condition is caused by the alternator since that is what your car runs off of, not the battery. I am not sure about the schematic of the datsun alternator, but I know a GM alternator will run for 10-45 seconds after you switch the ignition off if the "exciter wire" is wired in with the positive wire to the electrical system. This wire should be connected in a manner that it is only connected when the ign switch is in the start position, not in the run position. Otherwise the alternator will self-sustain itself after you turn it off. Quote Link to comment Share on other sites More sharing options...
Guest BigWhyteDude Posted August 9, 2003 Share Posted August 9, 2003 something to consider about cleaning out your engine. You might try changeing your oil and replacing it with deisel fuel and running you car for a few minutes. then change the deisel and putting oil back in your car. This really really helps to degunkify your engine. Is that a word? Any way we(as in me him and another friend of mine) tryed this on one of my older friends farm trucks and a LOT of gunk and crud came out with the deisel. It work really well. Some of you might think i am crazy but we did it and it worked so there ya go. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted August 9, 2003 Share Posted August 9, 2003 When you turn the car off all electrical power delivery to the engine is stopped by the ignition switch opening the circuit so ignition timing has nothing to with this. The car will diesel simply due to something too hot in the chamber or the octane being to low. You can try a colder heat spark plug but I doubt that is your main problem. Another thing that almost ALWAYS will cause dieseling in a Z with SU carbs is having your idle speed set too high. It's really no big to turn the car off in gear. BTW are you getting any pinging while driving the car? Later Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted August 9, 2003 Share Posted August 9, 2003 something to consider about cleaning out your engine. You might try changeing your oil and replacing it with deisel fuel and running you car for a few minutes. then change the deisel and putting oil back in your car. This really really helps to degunkify your engine. Is that a word? Any way we(as in me him and another friend of mine) tryed this on one of my older friends farm trucks and a LOT of gunk and crud came out with the deisel. It work really well. Some of you might think i am crazy but we did it and it worked so there ya go. Yes I think you are crazy. I would hate to see the condition of the bearings and cam journals after this procedure. There is an additive that you can buy at an auto parts store and add it to the engine oil for this procedure. later Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted August 10, 2003 Share Posted August 10, 2003 ']When you turn the car off all electrical power delivery to the engine is stopped by the ignition switch opening the circuit... ..unless the alternator is still producing power. Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted August 10, 2003 Share Posted August 10, 2003 ']When you turn the car off all electrical power delivery to the engine is stopped by the ignition switch opening the circuit... ..unless the alternator is still producing power. The ignition switch terminates power to the ignition circuit. When you turn the car off the alternator is still turning for a second so following your logic the car should never turn off. It's simply not the case,once you turn the ignition switch all the way to off there is no power going to the coil or distributor. later Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted August 10, 2003 Share Posted August 10, 2003 ] ']When you turn the car off all electrical power delivery to the engine is stopped by the ignition switch opening the circuit... ..unless the alternator is still producing power. The ignition switch terminates power to the ignition circuit. When you turn the car off the alternator is still turning for a second so following your logic the car should never turn off. It's simply not the case' date='once you turn the ignition switch all the way to off there is no power going to the coil or distributor. later[/quote'] If you bring your car to Sac, Ca I can wire your alternator to do exactly as I am saying. I can assure you that it is possible if the alternator is wired incorrectly. In order for an alternator to work it needs to be "primed" so-to-speak, I forget the actual term used. In order to do so there is a wire that routes through the ign switch in the Start position that basically excites the alternator in order to get it to start charging. (This is the part of the circuit that makes your little battery light turn on on the dash.) If this wire is connected in such a manner that it connects to the On position on the ign switch then the alternator will basically "self-prime" itself when you turn the ign off and will allow the alternator to continue producing power for a short time. I have personally experienced, researched, and corrected this scenario with my pickup when I built the engine-compartment part of the wiring from scratch. I would even be happy to scan a copy of the page in the engine electronics principles book that I learned about this from. (I'd just need to dig it out) Quote Link to comment Share on other sites More sharing options...
Guest norm[T12SDSUD] Posted August 10, 2003 Share Posted August 10, 2003 ] ']When you turn the car off all electrical power delivery to the engine is stopped by the ignition switch opening the circuit... ..unless the alternator is still producing power. The ignition switch terminates power to the ignition circuit. When you turn the car off the alternator is still turning for a second so following your logic the car should never turn off. It's simply not the case' date='once you turn the ignition switch all the way to off there is no power going to the coil or distributor. later[/quote'] If you bring your car to Sac, Ca I can wire your alternator to do exactly as I am saying. I can assure you that it is possible if the alternator is wired incorrectly. In order for an alternator to work it needs to be "primed" so-to-speak, I forget the actual term used. In order to do so there is a wire that routes through the ign switch in the Start position that basically excites the alternator in order to get it to start charging. (This is the part of the circuit that makes your little battery light turn on on the dash.) If this wire is connected in such a manner that it connects to the On position on the ign switch then the alternator will basically "self-prime" itself when you turn the ign off and will allow the alternator to continue producing power for a short time. I have personally experienced, researched, and corrected this scenario with my pickup when I built the engine-compartment part of the wiring from scratch. I would even be happy to scan a copy of the page in the engine electronics principles book that I learned about this from. (I'd just need to dig it out) How could the power get to the coil or distributor though? It can't jump an open cicuit. Later Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted August 10, 2003 Share Posted August 10, 2003 There was a thread about the alternator problem some time ago. There is some type of feedback loop where the alternator somehow feeds the ignition when the switch is turned off. I forget what would caused it to occur, but I thought is happened when someone replaced the alternator? I do remember the fix was to place a diode in the alternator field circuit so the current could only flow in one direction. BUT, in the thread I am thinking of the car would not shut off. It would keep running until other measures were taken to kill it. Dieseling is very different from a running engine. If the alternator circuit were at fault then the engine would continue to run like it had never been turned off. Diesels don’t need an ignition circuit, the heat from compressing air causes self ignition. So I would be surprised if an ignition issue is the problem here. Having driven a bunch of junky old 60's vintage cars, I can say that dieseling is definitely related to high compression and low octane gas. If the engine gets old and has deposits in the combustion chamber, then dieseling is much more likely because the deposits make compression that much higher. Deposits also get hot and can cause ignition. Any kind of vacuum leak will make the dieseling worse (basically anything that would cause a lean condition, especially if it is compensated for by enriching the idle circuit). Thus SU’s that haven’t been rebuilt will have vacuum leaks around the throttle shafts exasperating the dieseling problem. Driving an old 60’s car with 70’s gas usually meant leaving the car in gear and (if possible) turning on full choke when the key is turned to off. The only real fix was to rebuild the engine, clean up any deposits, and possibly lower the compression ratio. Modern cars don’t have this problem because the fuel injection turns off the gas when the key is turned off. I had an old car that had a solenoid in the idle circuit of the carb that would do the same thing. Quote Link to comment Share on other sites More sharing options...
Len Posted August 12, 2003 Share Posted August 12, 2003 Rebel I just gotta know which wire excites my alternator. I had a love affair with my 280 zxt for a few years now. Anything I can do to please her I would do! If it only takes a certain wire to get her excited then hey! I'll go for it. Because I feel nothing is to good for my babe! Sorry if I got carried away. Its just one of those "Z" things! Quote Link to comment Share on other sites More sharing options...
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