perattzi Posted October 15, 2014 Report Share Posted October 15, 2014 So I've been struggling with a strange temp issue with my truck. Seems troubles with the temp gauge are somewhat common on these old vehicles, but I'm not sure it's a gauge issue. It started right after I bought the truck a few weeks ago, my temp was gauge reading in the upper 3/4 of the hot zone and up to the first line even. Then a small coolant hose that was super old and crusty failed. So I replaced that, along with the sending unit, thermostat, and junked the old deteriorated radiator and cap for a new one. Flushed the coolant and filled with new. Adjusted the water pump belt. The fan clutch is good. Tested the gauge by grounding the wire from the sending unit, and it climbed to FULL hot as it should. Now when I drive it the temps will climb up to middle, sometimes stay there for a bit..but then climb to hot..and now will sometimes drop below the middle range. This afternoon when I saw the temps rise up, I pulled over and took readings with my infrared thermometer, the upper 180's and lower 190's are the highest readings from the sending unit and head areas. It seems when I am driving the needles raises but when I stop at an idle the temp is falling down. Maybe my new thermostat is no good? This leads me to my next issue. I live in Portland, Or and am subject the DEQ testing. I stopped by today to what would happen and I failed the CO test. The HC etc were well below the acceptable level and the the CO was 2.7 when the limit is 2.5. So not far off. I was planning on just lean out the idle mixture bc they test it at idle. But I noticed that when I had to wait in line, the temps were creeping down low. So maybe my engine just wasn't up to full efficiency when they tested it? Ill try letting it idle and get some temp readings then. Thanks for any help guys! Quote Link to comment
wayno Posted October 15, 2014 Report Share Posted October 15, 2014 What year is your truck, and what engine is in it? 1 Quote Link to comment
MikeRL411 Posted October 15, 2014 Report Share Posted October 15, 2014 Insufficient data yields very dubious responses, so let's provide some concrete data on the year, engine, transmission or original / replacement radiator! Your problem, not ours so let's provide the info needed to get some real life assistance here. Quote Link to comment
perattzi Posted October 15, 2014 Author Report Share Posted October 15, 2014 Sorry bout that, the truck is a 78 with an all stock l20b motor and stock 4 spd. I replaced the radiator with one I ordered from Advanced Auto Parts, aluminum with plastic tanks, direct fit in place or the crusty original. Quote Link to comment
wayno Posted October 15, 2014 Report Share Posted October 15, 2014 OK, so does your L20b have the thermostat housing with a bypass hose below the radiator hose? See the fitting/hose just above the vacuum advance housing. Quote Link to comment
perattzi Posted October 15, 2014 Author Report Share Posted October 15, 2014 My thermostat housing does not have that fitting/hose. Again today when I drove home from work the temp gauge would creep up while driving, then fall below the 'normal' mark to the cool side when I stopped and let it idle. I checked the temp at the thermostat and it read 160* the block read around 180-190*, this was maybe 2 min after it was just reading hot on the gauge, but now reading cool. Does a motor cool down that quickly? I also observed that the radiator hoses are stiff and under pressure, the upper hose is warm but the bottom hose is not, it is pretty cool to the touch, not sure how the system flows, I suspect from the bottom up? Quote Link to comment
wayno Posted October 16, 2014 Report Share Posted October 16, 2014 Hot water comes out of the top, goes threw the radiator and cools, then goes out the bottom hose to the water pump and is forced back into the engine block again. If the thermostat is not opening at the proper time, you will have wild fluctuations in water temperature, the question is why is it not opening at the proper temperature, there are a lot of opinions about this subject. I thought all L20bs had bypass hoses, so it sounds like you have an L16 thermostat housing on your engine, in my opinion, not having the bypass hose is bad. If you were to drill a 1/8th inch hole in the thermostat around the edge to let the hot water threw like the bypass hose does, it should fix your issue unless the thermostat is bad, but since it was doing the same thing with the old thermostat, I would suggest not having the bypass hose, or a hole in the thermostat is the issue. Some will say that the jiggle valve will fix this, but when I searched the purpose of the jiggle valve, it said it was to let air get past the thermostat to keep air from getting caught in the thermostat housing, it did not say anything about letting cooler water out to let the hotter water in the head get to the thermostat, IMO you should drill a hole in the thermostat where it will let a little water flow like the bypass hose does. My thermostat housing does not have that fitting/hose. Again today when I drove home from work the temp gauge would creep up while driving, then fall below the 'normal' mark to the cool side when I stopped and let it idle. I checked the temp at the thermostat and it read 160* the block read around 180-190*, this was maybe 2 min after it was just reading hot on the gauge, but now reading cool. Does a motor cool down that quickly? I also observed that the radiator hoses are stiff and under pressure, the upper hose is warm but the bottom hose is not, it is pretty cool to the touch, not sure how the system flows, I suspect from the bottom up? Quote Link to comment
perattzi Posted October 16, 2014 Author Report Share Posted October 16, 2014 Hmm interesting. Here is a photo of the thermostat housing on my engine It has some sort of outlet with a hose right below the sending unit, the hose then connects to a hard line that goes to the heater hose on the lower right side next to the block. Also on the left side of the thermostat housing there is a small chunk of metal missing on the outside of the threads where the thermostat cover/outlet bolts on. Somebody must have torqued that bolt down too hard at some point and broke that off. Only a few threads engage but I figured it would be fine. I used a bit of sealant on the gasket but it seems I may need to retighten and see what happens bc I seem to have tiny leak there, it's a bit moist. I think I may try and source a new housing and in the meantime take the thermostat out and test it/drill a hole/replace if necessary. Thanks for trying to help Wayno I really appreciate it. Quote Link to comment
DanielC Posted October 16, 2014 Report Share Posted October 16, 2014 Who knows what the previous owner has changed as far as was thermostat housing was originally on your truck. I am a firm believer in using OEM (Nissan) parts on Datsuns. Go to Gladstone Nissan. Talk to Andrea in the parts department, and tell her that you need to look at the parts that are should be on the thermostat housing of your engine. Get a new Nissan thermostat while you are there. You might have to order it, or they might have it. If you are missing the right thermostat housing, you may need to get the correct one for your engine. Pick-n-Pull is one option to get the correct parts, posting here is a another option, or you can see if the part is available from Nissan, while you are at Gladstone Nissan. Say you are a member of Community.ratsun.net, and Gladstone Nissan will give you a discount on parts. Quote Link to comment
perattzi Posted October 16, 2014 Author Report Share Posted October 16, 2014 Thanks again for the replies, the pick n pull database says theres no 620's near me, but I supposed any l20b with the same would work correct? The tiny leak is only present when the motor is cold, after stared/driving it seals so maybe I just need to retighten the thermostat outlet cover. So I tried the DEQ again. Failed again! But for different reasons this time. When they test they just put a sniffer in the exhaust. They test at idle, then rev to about 2500rpm then retest when idle drops again. The first time I went my HydroCarbons readings were 89 first idle and 97 the second, the standard is less than 300, so I was good there. However, I failed the CO level, mine were 2.7 while the standard is 2.5. So i was super close the first time, but not close enough. So this time before I went I drove the truck for about 20 minutes to warm it up, then I removed the air filter and turned in the idle mixture screw in til it was stumbling quite a bit, then raise the idle mixture so it would idle good enough. Then I drove it around revving quite a bit, the temp gauge wasn't going up high like before... then I had to wait in line and idle for like 15 minutes, and of course the engine temp dipped like before. This time my result were: HC #1 1112 HC #2 617 CO #1 .21 CO #2 .22 So, obviously leaning out my idle mix lowered the CO dramatically. But why did my unburned HC got up so much? I'm guessing it is due to the low idle temps, like my thermostat is sticking open? Quote Link to comment
DanielC Posted October 17, 2014 Report Share Posted October 17, 2014 High HC is unburnt gas. By leaning out the engine, you created a condition called "lean misfire" A good way to set idle mixture is to get the engine warm, and hook a vacuum gauge to MANIFOLD vacuum. Make sure the ignition timing is good, and adjust the valves. Tune the idle mixture screw to maximum vacuum at idle. Make sure the idle speed is kept down, too much idle RPM, and mechanical advance starts coming in. Some engines you need to disconnect the vacuum advance, others you do not. Once you have maximum vacuum at idle, turn the idle screw in (leaner) to drop idle speed about 50 RPM. See if that works to get you through DEQ. 1 Quote Link to comment
perattzi Posted October 17, 2014 Author Report Share Posted October 17, 2014 High HC is unburnt gas. By leaning out the engine, you created a condition called "lean misfire" A good way to set idle mixture is to get the engine warm, and hook a vacuum gauge to MANIFOLD vacuum. Make sure the ignition timing is good, and adjust the valves. Tune the idle mixture screw to maximum vacuum at idle. Make sure the idle speed is kept down, too much idle RPM, and mechanical advance starts coming in. Some engines you need to disconnect the vacuum advance, others you do not. Once you have maximum vacuum at idle, turn the idle screw in (leaner) to drop idle speed about 50 RPM. See if that works to get you through DEQ. Ha yes you are correct that must be exactly what I did. Probably could have just leaned it a smidge and had been ok, but went overboard. It was idling rough and shaking a bit. A vaccum gauge is something that should have but do not, I need one for tuning bikes anyhow. Thanks for the tips I will check the basics as you have stated. As for the engine temps... still a mystery. When I drove home from deq the gauge was reading pretty will in the middle the entire time, except dropping a bit at idle. Then I turned on the heater just see what would happen, to my surprise the gauge needle went HOTTER. Anyway, for good measure I went and bought a new 180* thermostat and tested it in a pot of water on the stove with a cooking thermometer and my infrared to verify, it opens and closes as it should. IN addition to installing that I changed the oil/filter and spark plugs. The p.o. had just put new plug wires on. But I will double check timing etc. I really just want to sneak this thing by DEQ any way possible then legimately fix any and all problems. It's just a time issue bc I only have a trip permit to get around in it until I can register it. BAhhhh Quote Link to comment
wayno Posted October 17, 2014 Report Share Posted October 17, 2014 You have the hose on your thermostat housing your supposed to have, that is the bypass hose, so you are good there, it may not look like mine, but it is the same thing, a bypass hose. When it is warmed up, take off the radiator cap and look in the hole, you should see the water moving. The water temp on my 521s reads about in the middle, when on the hiway going up a hill it gets a little past the middle, and when I pull off the freeway it stays hot for a little while till I get threw all the signals, then it cools down, but once warmed up, it never drops into the cool area like half way to the middle. Quote Link to comment
jastrunk97 Posted October 17, 2014 Report Share Posted October 17, 2014 BE CAREFUL WHEN REMOVING THE RAD CAP!!!! Ended up in the ER the other day with 1st and 2nd degree burns on my entire chest. Not fun Quote Link to comment
Fat510 Posted October 17, 2014 Report Share Posted October 17, 2014 BE CAREFUL WHEN REMOVING THE RAD CAP!!!! Ended up in the ER the other day with 1st and 2nd degree burns on my entire chest. Not fun That's a rookie mistake there man lolz Quote Link to comment
perattzi Posted October 17, 2014 Author Report Share Posted October 17, 2014 BE CAREFUL WHEN REMOVING THE RAD CAP!!!! Ended up in the ER the other day with 1st and 2nd degree burns on my entire chest. Not fun Damn, sorry to hear that. I bought a cap that has a pressure release lever. Kinda nifty. Quote Link to comment
perattzi Posted October 18, 2014 Author Report Share Posted October 18, 2014 Ok I got my truck to pass DEQ! Just adjusted the idle mixture to where it was revving the highest then brought the idle speed down and then leaned the mix just a touch. Quote Link to comment
wayno Posted October 19, 2014 Report Share Posted October 19, 2014 Success feels good, doesn't it. :) Ok I got my truck to pass DEQ! Just adjusted the idle mixture to where it was revving the highest then brought the idle speed down and then leaned the mix just a touch. Quote Link to comment
perattzi Posted October 20, 2014 Author Report Share Posted October 20, 2014 Yes indeed! Now just to solve the cooling mystery. So in order to test that the temp gauge is working correctly I need to use a resistor correct? Do I put the resistor on the wire after pulling it off the sender then ground it? Thanks. Quote Link to comment
DanielC Posted October 20, 2014 Report Share Posted October 20, 2014 Yes, pull the temp sensor wire off the sender. For a 521, the value of the resistor is 40 ohms, to read half scale. ground one end of the resistor, connect the temp sender wire to the other lead on the resistor. The value of resistor may be different for a 620. Quote Link to comment
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