ivyleague

First, I want to thank those who post regularly and have provided me with some great information about the 300zx that I would've never known without the forum.

Here's my story, I got a "great" deal last September on a 85 300zx non-turbo M/T for my son's 1st car. On the way home, the efi fusible link fried and melted a bunch of wires in the harness plus the ecu.

And that's when my nightmare began, I was able to get a working ecu (after 2 attempts) and was able to get it running (not great) for a total of a week when the ecu started giving me every code I think Nissan ever created.

Using it as a learning lesson for my son and I, we decided to go remove the harness and testing all the connectors and wires and replacing the ones that were no good, along with testing the MAF (which was bad and required replacement) and TPS (which we ended up stripping one of the bolts and had to get a new Throttle Body). We also got a couple of grounding blocks and ran anything that was a ground to it.

We've also replaced the rear subframe bushings (while still on the vehicle since we couldn't get one of the crossarm bolts off, PITA but it can be done).

We had to re-route the coolant lines to the Throttle Body as they were different.

We put the harness back in and throttle body back on and were set to fire it back up to celebrate my son's birthday except....the efi fusible link keeps frying the instant I turn on the key and when I pour coolant in it, it runs back out from someplace behind the plenum. The coolant is coming from between the back of the oil pan and flywheel (it actually seems to be coming out of the hole on the bottom of the flywheel but that can't be possible can it?)

It's only got 100,000 miles on it and when it did run that brief time it was like a narcotic, a high that keeps you coming back for more. But, I'm about to go to rehab and just light the thing on fire.

I will eventually figure out the electrical part, but I'm hoping someone can provide some guidance on the coolant leak, because I can't find where it's coming from.

Thanks in advance to all that have read my sob story and offer their advice

zxguy1986

Welcome to ZDriver! Great Z31 you bought. You are catching up with work that has needed to be done on it for a while. You're right that they are an addiction - but definitely worth the work.

For starters, you have a copy of your Factory Service Manual (FSM) - or you can download it from http://www.xenonz31.com/reference.html. There are other good docs there, too. Take some time to get familiar with the FSM and check out the inspection & troubleshooting sections for the stuff you are working on.

The pros here will have some questions for you, no doubt. Again, welcome to ZDriver!

PredatorZ

Most likely the PO has done some additions or alterations to the wiring, most commonly in stereo installations or alarm system. Look at the FSM, isolate the circuit that is frying Links and it might come down to stripping the harness down and checking out each wire. If you do any repairs, use solder and shrink tubing for a quality lasting repair. If you don't have a multimeter, buy a cheap one at harbor freight. A Fusible link burning is keeping your harness from melting, so someone has reversed a ground or hot wire, or tied them together. The harness is color coded, so tracing wires is made a lot easier. Good luck and have patience !

Make sure your grounding block is tied to the negative terminal of the battery, the body/ frame alone is a very poor ground.

ivyleague

Thanks for the advice on the fusible links, I thought I took my time and checked everything when we took the harness out before. I've obviously got something mixed up.

I should've mentioned before that I have the FSM and have used it extensively to check out the harness (again, not good enough) and for the ECU and MAF. But I'm stuck as to why that when I turn on the key the fuel pump won't turn on and melts the fusible link.

But I'm trying to find out where the coolant leak is coming from, I put paper towels in behind the plenum, behind the throttle body, everywhere where there was an opening and the paper didn't get wet. I haven't had any luck finding info in the FSM about this. I really hope I don't have to tear into the plenum.

zxguy1986

All yeah to what PredatorZ said above. Somebody has screwed around with your harness in the past and they messed up

A melting link is serious but has to be something elemental. I mean that your whole harness isn't conspiring together to melt that link - it's a single crossed feed somewhere that needs to be reversed. That you should be able to track down with your FSM schematics. Yeah, it's a pain, but they are right. Multimeter!

Your fuel pump turns on when your key hits pre-ignition, dash warning lights all come on - and you can hear the pump humming. Then ignition and you start - partly because the fuel pump had a head start in getting pressurized fuel to your injectors. If you can't hear the pump running at pre-ignition, fix that first. (Just remembered that the only time I ever had a problem with Z31 fuel pump performance was when my ECU was bad... I had to hard wire the pump until I solved it)

Coolant leak is a tough one. I have found slow leaks from everywhere over the years, from hoses, clamps, soldered joints, pipe cracks, radiators, drain plugs, etc. Not to mention the dreaded gasket leak... Yours sounds like more than a slow leak. No surer way to track it down than put the car on a lift, jack it up, full radiator, cap on, start car, let it run and run. Stand underneath and watch. Lots of light underneath and flashlights in hand. Just watch, you will eventually see the drip on the floor. Look up.

I am suspicious of your ECU switch-out and the wacky codes you got. How do the part numbers on the ECU you put in (and the original ECU you took out) square with this resource from an earlier Z good guy? resources:z31_ecu_list [Z31.com] Make sure you have the exact ECU part number for your model. I don't think you mentioned any mods that would have invalidated your orig ECU.

Report back as you plow ahead. All input you have is valuable. You are tracking more than one prob at a time so guys here tend to hold back until you get a handle on at least one... We just prefer to focus...

86tcoupe

The coolant leak I would hook up a radiator pressure testor pump and have some one pump it while you listen for the hiss to pin point .
the melted wire problem could be harder to solve, more a problem for a pro but how melted are we talking ? pulled them apart? it likely melted another section in a inexcessable place. if your nose is great you usually get a strong burnt smell near it.
there are circuit breakers you can hook up and short detectors ,but hard to use on bundles
also you can have a relay that the feedback control system has gone hay wire , so try unplugging some to see if it changes


PS: that car will do 155 mph like others do 80-90 .... 1st car huh

ivyleague

Thanks all for the advice.

I haven't had time lately to look at the car for awhile, but to answer some of the idea's and suggestions brought up.

The previous owner had the car repainted and must've just covered the engine with a tarp as some of the wiring harnesses have paint all over them. I bought the car from a guy about 120 miles away from my home and the car started to run bad about halfway home and by the time I got to town I could barely keep it going. So I took it straight to the Nissan dealer to have it looked at and they said that the ECU was fried and to "have a nice day". (I had planned on having them look at it anyway because I was hoping that the injectors were pre-recall and they would swap them out, no luck, the previous owner had actually done it already).

So my dad and I towed it home and we started digging into the harness and it didn't take long to find out what happened. The fusible link from the efi relay was melted from the fusible link box to the firewall going to the computer inside the car.

So I bought the same type ECU as the original (#24) the one for 5sp MT non-turbo. My dad is an electrician so he bypassed the fusible link box with a old-school fuseblock and we ran new wires for the efi relay to the ecu. We checked and rechecked all the connectors for continuity and ground and they all checked out so we hooked up the battery and it worked, we got it running but not great. It would bog severely when I pushed on the gas but would idle great.

I wanted to run it so that it might throw codes, but it never did. The ECU never had the green and red bulbs flash at the same times so I suspected MAF, got a different one of those and still no red/green bulbs. I went through the FSM for all the diagnostics of what could cause the bog, checked fuel pressure = good, changed CHTS, changed fuel filter. No difference. After many of the diagnostic results kept coming back "bad ecu" I got a 3rd ECU, (#24 again) and was able to get red/green light but I was never able to get them to go on @ the same time. Still the bog, I then tried new battery cables and grounding blocks, still the bog.

I tried to adjust the TPS and ended up stripping the screw in the throttle body and having to get a new one of those, which was different than the original so I had to modify the coolant lines through the throttle body (I'm thinking I did something here to cause the coolant leak somehow?).

I decided to get a used engine harness from 300zxpartsforyou.com because I think there was some gremlin or something in my old harness that I'm missing so I got the new harness and went through and tested everything, replaced a few bad connectors. I just think I had some wire(s) in my old harness that were damaged from melting in the beginning.

So we slapped the harness in, and that's when the blowing fuse before the fuel pump engages started happening.

I would pressure test the cooling system but the coolant literally runs out the back of the plenum as soon as I pour it in so I will never be able to fill it up until I find where it is leaking from.

Sorry for the long post and reading this far.

zxguy1986

I guess we have to take your new post as progress. Your 'great' deal on a Z turns out to need some serious work.

Truth is, like any used car, they all do. You're making progress and dealing with the frustration. When I went through it - more than once - I kept reminding myself that it is, after all, only a machine. If people originally put it together and it ran right, people can make it run right again. It's just a very large number of used car parts trying to work together. I was only there to help.

ivyleague

I really do appreciate the replies, wasn't trying to offend, and you are right it is only a machine.

I plan on working on it over the weekend and will post any progress that is made.

ivyleague

Well, finally had time to figure out why the fuse kept popping for the fuel pump. It was a stupid mistake, I had grounded a black wire that was actually a hot wire.

Now, it turns over but I'm getting no spark. I went through the FSM and it turns out the ignition coil was bad. Ordered a new one and still no spark. I really hope I don't need another ECU.

I'm going to try a new cap and rotor, but am not holding my breath that this will fix it. Open to any suggestions.

Still can't find the coolant leak. I'm concentrating on getting in running first and getting it to cool once I need it.

Thanks in advance one and all.

ivyleague

Well I found the coolant leak, there was one little coolant hose that was only halfways on. Put it all the way on and now the leak stopped. It was on the plenum on the passenger side of the engine.

As far as the spark issue, still haven't got it. I'm now leaning towards the power transistor, I've going to try the PRW2 mod and see if that does it. Otherwise it's another ecu.

This being my 1st Zcar, is it normal to go through ECU's like this? Is there a way to fix them? I've got 3 now and I think my last one is good, but all the tests in the FSM come back for the "no spark" issue with "replace ECU".

Also, is there any other ECU's that would work for my car? Mine is an 85 5sd, analog dash, non-turbo. It came originally with the ECU #24 and that is the one that I've been getting as the replacement, but if there is a better option then please let me know.

PredatorZ

I don't think its ever normal to go through ECU's, but multiple issues on a car can be difficult to fix. How are you testing for spark ?

rogerz

it is also possible that that crank angle sensor (which is inside the dizzy) isn't sending the signal to the ECU. The ignitor is just a power transistor you could probably buy one from Radio Shack for cheap and jury rig it to work to see if yours is dead. You can also look on the net and look for ways to test transistors with a VOM. Not a hundred percent that it will work but you can for sure tell if it is dead. short story. base to emitter low impedance, base to collector low impedance. collector to emitter high impedance. If you are electronic savvy you an pull the cover and looked for burnt connectors and/or components on the circuit board. I was able to cobble one up to work after putting a piece of wire to replace a board connector and replacing a fried diode. pretty sure it is a double sided single layer board given the date it was made. also check on line for ECU compatible with the O2 sensor. I dimly remember 85 was an oddball. do a net search.
Like PredZ said how you checking for spark? grab a wire and feel the jolt or using a GROUNDED spark plug and observing what happens. Those are the most surefire methods but the jolt gets a little old. You can get a noid light cheap to see if the injectors are getting the go signal. I wouldn't put much faith in the trouble codes. they might have been some help when everything was new and shiny but i have found them sending me down the wrong path more often not that stuff is old and tired. what tests made you assume the spark coil was bad? you can hook up a plug wire and plug (GROUNDED WHICH A LOT OF PEOPLE NEGLECT), hook primary to ground on one terminal then bump the other with 12v (Make and Break is how it works what points used to do). You should get a spark if the coil is good, Are all your spark plug wires good? Is your battery fully charged and tested under load? low voltage on a Z31 is a killer. Alessandro Volta checked that out in the late 1700 I believe. Be sure your battery cables are in A one shape. Clean Tight terminals not those clamp on on size fit all pos things. Be surprised How often i look at a z31 that got all sorts of gremlins and in the end bad battery or bad cables. Heart of your electrical system right there. Very often neglected or overlooked as a problem area. I see you got new batt cables but did you put in new wire with clamp terminals or did you go to a battery shop and have lead sealed new terminals crimped on or a complete new cable with terminals made up already.

ivyleague

Predz & Rogerz,

Thanks to both of you Z veteran's for taking the time to respond. I've appreciated your input, not only on this post, but for all of your other posts as well, always good advice.

As for checking for spark, I did mention my son right? I've been having him hold the plug wire while I cranked it over and when it didn't knock him over I assumed no spark.

All kidding aside, I took the #2 plug out and set it alongside the plenum to see if it sparked. I guess I was figuring since it worked on my 68 Mustang it had to be the same for a Z. I'll get one of those inline spark testers and maybe a noid set and do better diagnostics and go from there.

As far as the coil, I went through the FSM (EFEC pgs. 59-61). The FSM wasn't very clear about the power transistor test, I couldn't get a consistent reading on it. So I went to the coil and did find that the readings weren't on spec's so I replaced it.

I must confess that I don't have adequate battery cables as of now. The battery however is brand new but I haven't had it load tested. Rogerz, what cables do you suggest I get?

Thanks again gentleman

ivyleague

FINALLY!

I know I always appreciate it when people list what worked for them to fix their problems even if it is months after the original post

After many months of frustration I've figured out the spark issue. It was the connector for the CAS. I replaced it and got spark.

It runs, but I think when I changed the CAS itself I got the distributor off a tooth or two as it spits and sputters. So now I'm on a mission to find TDC.

Also, when I did have it out on the road I noticed that my replacement of the subframe bushings did not work. I guess I'll have to replace the differential mount insulator.

Stay thirsty my friends.