Hi guys, I recently bought a Cedric that had a cooked engine (VG30DE) and decided to fix it up. Since it has a similar engine to the NA 300ZX i thought you guys might be able to help me find this curious overheating problem So the first step in fixing this car was to buy another VG30DE engine, and then at the suggestion of my mechanic, replaced: Water pump Timing belts, fan belts Engine gaskets and seals (except headgasket) All hoses (inc radiator hoses) Radiator (replaced with a Pathfinder radiator, which was similar size - radiator shroud still in place) Spark plugs Thermostat The heater tap was seized so we are ordering another one, at the moment the heater is bypassed. But then late one evening a couple of weeks ago it overheated while i was coming home on the M5 (after doing 90kmh+ at 1800RPM for 10minutes or so). I only just noticed as i pulled off the motorway, and i promptly started panicking. But then i noticed the temp started going down as i limped to find a safe place to stop. My house was only a km or so away so i kept going, and by the time i got home it was at normal temp again. Back to the mechanic... After some stuffing around i put a new clutchfan in there (its a 300ZX TT clutchfan, but it seemed to work for the next 2 weeks) Until tonight. I was on a side street and decided to gun it to make it through a traffic light. I tend to baby this car so this was the first time i had the revs over 5000. Then about 30 seconds later i noticed the temp gauge getting to about 80%. Pulled over and waited until it went down again. Then for the rest of the night it was constantly trying to edge up past halfway, with no set pattern to it. It would start getting hot whenever i got over 70kmh even though the engine revs are extremely low at high speeds (under 1500RPM) At traffic lights it sits rock solid on the temp gauge At random intervals while driving it suddenly started creeping up again. Oh and after shutting off the engine, i hear the obligatory bubbling sounds from the radiator/overflow tank area... I've read the A-Z of Cooling guide, and the only thing that i can think of is air pockets or obstructions in the block cooling passages. I'll put it in for a power flush tomorrow, but any other ideas? It just seems so random and i dont know why it overheated on the motorway (at higher speeds...)
Sounds like you may need to pay attention to the coolong system Probably the best thing you could do is to remove the radiator and take it to your nearest recognised radiator shop and get it "rodded". This should improve the flow through the radiator. Also check that the thermostat is functioning properly. In reference to the temperature guage, do you have an after-market guage on the Cedric? The OEM guage that comes on ALL Zeds is notoriously inaccurate... seems to go OFF, ON, BANG!
Re: Sounds like you may need to pay attention to the coolong system I only just replaced the radiator with a brand new one... Would it still be necessary to rod it? And its only the stock temp gauge. I am building a consult interface so i can see the exact temp, but i think i need an aftermarket one too
Maybe a head gasket. You need to get a compression test done, and also an analysis of the coolant to see if there are any hydrocarbons in the water.
Re: Seconded Thanks, I hope its not the HG. Its the only gasket my mechanic didnt replace when fitting the engine Luckily i did buy the VR-S or whatever its called so i do have the headgaskets, but i'm guessing its not a cheap thing to fit! One thing to note is that there is no white smoke from the back of the car, in fact no smoke at all; I thought HG problems always presented as billows of white smoke out the exhaust? Assuming that even smokeless cars can have HG problems, can the coolant analysis be done by my mechanic? Or should i take it to a radiator place? (i know he has the equipment for a power coolant flush, which I will also ask him to do) I dont want to sound like one of those guys who just takes it to the mechanic and doesnt do anything by themselves - its just that i paid alot of money for my mech to fit the engine, so he's doing alot of this stuff for free for me now. At least now i have some suggestions which i can ask him to look at when i see him...
Re: Seconded Not always white smoke in exhaust. I had a VR commodore that ran for years with a leaky head gaskett. Just overheated every now and then and used water. Knew what it was and confirmed it when the timing chain busted and I pulled the head off. Obviously had been leaking for ages. But ran fine otherwize. The fact it has been cooked seems to lead me to ask the question, as the H.G. is a failure mode in overheating as is a warped head, Both can make the H.G. fail
Re: Seconded Ahh, well in a way i hope thats the answer because i just want it sorted out and fixed. I dont like driving with my eyes glued to the temp gauge (which is not even supposed to be accurate!) I left the car with the mechanic and asked him to do a compression test + the hydrocarbon test like you said. If its not that i asked for a power flush, since that wasnt done yet. I guess i wont be driving the car up to Coffs Harbour this weekend! Edit: Warped head? That sounds painful and expensive, lets hope its not that!
Re: Seconded Well, i got it back from the mechanic and he couldnt find anything immediately wrong with it. He said the radiator fluid had a fair amount of rust in it but he didnt think that would be the only cause of the overheating. He flushed the coolant and told me to go for some long drives and see how it goes... The cylinders all had good compression (engine is only supposed to have done 22,000kms), and he didnt notice any oil/petrol in the coolant (didnt have it properly tested though. He also suspects headgaskets, but there really isnt much evidence other than the random overheating. Doesnt use coolant (not that i can notice anyway), doesnt smoke... And he said changing the headgasket is a big job on these cars - he said it could cost more than $1000! I already have the gaskets, should labour cost that much for a HG change (keep in mind i have a bit more engine room than a Z)?
head gasket failure is rare with these engines, you can get a CO2 test done at a radiator specialist to be sure though. I also wouldn't trust a regular mechanic to properly flush the engine, there could be a partial blockage in there somewhere if rust was found. I'd also be suspicious of the replacement water pump.
Re: head gasket Ah... good idea. I will take it to a Radiator place since i have it here with me. Good to hear VG headgaskets are tough. I'm starting to worry about the waterpump too - it was supposed to be a new one, but perhaps it was secondhand or just crappy. I just got a Davies, craig electric fan (16") on ebay so i will be putting that in front of the clutchfan when i get the chance...
Howdy Dom! Get the radiator guy to pull your radiator and rod it. This takes the end tanks off and clears all the core. My radiator was relatively new but was still 1/3 clogged - Not good hey. Its pretty cheap and they can also pressure test your cooling system to find leaks as this is a pretty common cause of this stuff. Read Marks (aka Zed4lifes) cooling system article in the tech section, it is the bible for our engines. They are tough engine as I have nasty things temp wise in the past to mine and come off ok:wacko:. The 300zx temp gauges also always sit on the halfway mark, cant tell you about the cedric. As you know they are crap gauges though. MattB
Overheating... Running air con can push up temps; also easy check for head gaskets probs is to take off radiator cap WHEN COOL, start engine and watch for air bubbles in radiator fluid...
Re: Overheating... Ok, did some more testing today. Drove it to and from work (45mins each way in peak hour traffic) and then to campbelltown on the M5 (30km or so on the freeway each way), and a detour through liverpool too. I put over 100km on the car today ranging from bumper-to-bumper, to 120km/h on the freeway, and the temp gauge didnt go over the halfway mark The temp gauge actually started sitting LOWER than halfway at one point, but it started getting a bit jumpy and erratic. Guess its not something that should be relied on, but at least it didnt overheat! I think all OEM temp gauges only have 3 positions though But I had a think about it, and I am pretty sure that both times it overheated was right after i had put the boot in and accelerated through to redline (or as close as the auto would let me). The first time it redlined, i had booted it joining the motorway, and i might not have noticed until i got off and it was nearly overheating... Could that point to a HG problem? I am probably not going to be able to bring it to a Radiator place until next week...
