280z Backfires, can't find problem

[Phillip]

I've been working on my 280z for coming up on 2yrs, and ever since I got it running, (last december) it has been backfiring, have yet to figure it out, my dad was the only one to drive the car when the car was first bought, and he doesn't remember if it did....of course.

 

I was told when I was getting my exhaust done that my car was running really rich, then after that, I was told by my friend's dad who is a mechanic, so I bought it into a shop to get it diagnosed and adjusted, and they found it to be the air fuel mixture as well, so he tried to adjust the air flow meter, but when he leaned it out it would just backfire (I had the plug off of the MAF Sensor, but since I have spliced in a new one) Now according to my FSM, and the last shop the mixture is adjustable, the mechanic said it was set to full enrichment, So I called a local Z Specialist, and he said that the the air fuel mixture isn't adjustable, and I would have to buy a new one, and that it was probably more than just the air fuel mixture. So are they adjustable or not? and where is it adjusted?

 

lol now the problem is, when the car is cold no matter how to drive it it doesn't backfire, a little hesitation but not a lot, although when the gauge starts to register temperature, the car will backfire, but only when not driving hard, if I floor it, there is tons of acceleration, and no backfires at all, although if I am driving like im stuck behind a grandma, it backfires like crazy. The only mods are a K&N Air Filter, and a 2.5" cat-back exhaust

 

Any Thoughts? It doesn't backfire in neutral, it does hesitate though when cold in neutral

[Drax240z]

It's probably not a matter of adjusting the air fuel mixture. It IS possible, in small doses, by modifying the position of the AFM's toothed wheel. I suspect however that didn't get you to this point, and that won't fix the problem for you either.

 

Using your FSM, check the coolant temp sensor output, and also the two bullet connectors under the fuel rail from the coolant temp sensor. They notoriously corrode and don't conduct the correct signal, leading to a rich condition. If that fails, keep going through the sensors, cleaning contacts and checking outputs.

[Phillip]

Ok, thanks I'll check it out tomorrow morning.

[Phillip]

Ok so I replaced all the bullet connections under the fuel rail with butt connections (had a box of them) drove the car, and it takes a lot longer for it to start backfiring, and it doesn't do it nearly as much, it took prob 10mins to start backfiring, instead of like 2-3mins, about to check the coolant temp sensor output, thanks sooo much lol,

[dhp123166]

I would check the TPS as well. If it is malfunctioning it can alter the air/ fuel mix.

It's easy to fix.

[Phillip]

Ok thanks I'll check that out as well, is this the coolant/water temp sensor, you were talking about? http://www.thezstore.com/page/TZS/PROD/SCC13/16-7110KI

[Drax240z]

As I recall, there are 3 in the lower thermostat housing on the 280zx. (it's been a while) One for the ECU, one for the gauge, and one for the fan. You want to check the ECU one... I couldn't say for certain which one is linked, as I haven't had one out in quite a number of years. FSM!

[Phillip]

Ok, so the 3 sensors, are the Water Temp Sender, Water Temp Sensor, and Thermo Time Switch, and z-store says thermo time switch controls the Cold Start Valve which means its prob tied in with the CPU, i'm guessing if the Thermo is messed up the cold start valve would be on all the time causing it to become really rich? I couldn't find anything on the sensors in the FSM went through everything and it only said how to remove the thermostat housing. Anyone know which one is which?

[cygnusx1]

Make sure your injectors aren't leaking, your fuel pressure is perfect, your O2 sensor is working (if you have one), and that you have no vacuum/exhaust leaks before you get too deep. Also note that FI, Z's with large open exhausts tend to make pops in the exhaust during decel, in my experience.

 

You should check all three of those sensors with an OHM meter against the factory manual. You can check them right at the ECU connector to also test the wiring circuits that lead to the sensors. I use a needle to poke into the connector, and then clip my meter onto the needles.

 

Idle mixture is adjustable via an air bypass port in the side of the AFM. It mainly effects idle mix only. Running mixture is not adjustable, asides from hacking the AFM tooth wheel preload. That only effects the cruising ranges. The higher load ranges are pretty much unadjustable, unless you play tricks with the readings from the sensors, have bad sensors, or modify fuel pressures.

[Phillip]

ok, I will look for some more leaks I know of a small exhaust leak, coming off the EGR Tube, just need to put some sealer on it,

 

Also my dad has been bugging me since I got my exhaust done, to get it smogged, and if it failed I could go on the CAP thing, (500bucks for emission-related issues) So I took it today, and the thing passed smog, so idk people have been telling me it's running really rich, but if that were true, it prob wouldn't have passed right?

 

Cygnusx1, it doesn't backfire when decel, only when softly accelerating, and the car like pulls back then jumps forward,

 

If anyone in San Jose can stop by and check out my Z that'd be awesome, the sensors/switches sounded like a real good reason for it being rich, but it passing smog has stumped me lol

[woldson]

You have a cat on the car? Did you do anything to help your car pass?

[woldson]

Clean the trace lines on the afm and contacts on the tps. Also adjust the tps.

[Phillip]

there is a brand new cat (well was put on in march or so) I didn't do anything to help pass, I was actually hoping for it to fail, so I could do some CAP thing that I get 500bucks for emission related problems

[enix187]

Did anyone ever solve this problem? Im having nearly identical issues with my 83 zx 2+2, slight intermitent puff at idol, then radical popping (backfire?) under initial exeleration, and no pooping at cruise unless im in fifth. The pop is only noticable under light load, ie granny driving, if i get on it it doesn't do it, at least not as bad. The last bastion of hope i have is in my vavles, it sounds like a diesel, would that cause this?

 

edit: non turbo, stock motor, new intake/manifold gasket

[Phillip]

I haven't solved the problem yet lol, but I did however accidentally leave my thermotime switch and temp sensor unplugged when I went to get gas, no backfire at all. realized that they weren't plugged in at the gas station and plugged them back in, backfired like crazy afterwards. Needless to say I am pretty sure it's them.

I bought new Thermo, Temp Sensor, and Temp Sender, as well as a new thermostat housing. (as well as new connectors)

 

I'll know sometime next week when I get everything back into the car

[enix187]

Might have to try that, i've been fiddling with my afm non-stop. I can go from 10 gears rich to 10 gears lean, absolutely no difference in popping. I think timeing is going to be my next venture. But i might go try that now.

[enix187]

Well found my problem, injectors were CLOGGED, big time, and now they have blown out my hoses. So yay! found the problem, and bump me. This Z is a lost cause, bout ready to give up on it completely, just get another one.

[Dtsnlvrs]

I haven't solved the problem yet lol, but I did however accidentally leave my thermotime switch and temp sensor unplugged when I went to get gas, no backfire at all. realized that they weren't plugged in at the gas station and plugged them back in, backfired like crazy afterwards. Needless to say I am pretty sure it's them.

I bought new Thermo, Temp Sensor, and Temp Sender, as well as a new thermostat housing. (as well as new connectors)

 

I'll know sometime next week when I get everything back into the car

 

Unplug the thermo time switch, and the cold start injector...yoo live in CA for God's sake, you dont need it.

[Phillip]

Well I have since replaced my Thermotime Switch, Water Temperature Sensor and Sender. Not getting the full 12Volts through the wiring on the Temp. Sensor only getting 10volts, Thermotime Switch has 0volts going to it. and well the sender has always worked haha. Long and the short of it that did pretty much nothing except tell me that alllllll my wires are corroded.

 

Just started my engines class at my school, and been working on my car in there this whole week, did a compression test on tuesday, when I pulled plugs all of them were white, usually the plugs had some white/black around them, but now they are just white, which indicates running lean. Couple weeks ago I replaced my TPS because it tested bad, and when I replaced it, car still backfired a little bit, but I had a ton more power especially 3k-5k rpm. Which stumbled me because the car smells like its running rich but in the plugs, the replacing of the TPS giving more power all point to running lean.

Compression results were Cylinders 1, 2, 3, and 5 150psi; Cylinder 4 140psi; Cylinder 6 170. Going to be adjusting my Valves tomorrow, and doing timing. Then another compression test to see if it helps level them all out.

 

I was told to put some Fuel Injector cleaner in my car (which I had done maybe a year ago) and the guy told me to cruise at a higher RPM to clean out all the crap from my valves and everything and it should clean up the engine real nice and it'll stop running rough. Well Last night going to and from work, at times on the highway I would cruise in 4th instead of 5th, RPMs were about 3500, and I did the same on the way to school. and probably about a mile or 2 away from school the car actually stop backfiring when gently accelerating. There was still of course an absense of power feels kinda like the engine is just revving and no power, but once I go to where Full throttle enrichment hits theres a ton of power, let off a little its gone.

 

So im completely positive that the injectors are clogged real bad as enix187 mentioned before and im going to continue to use the fuel injector cleaner for awhile.

 

For the 'lack of power' on regular enrichment driving what can be the issue (I haven't tested the fuel pressure yet, yes I know it's stupid that I haven't done it yet Im gonna see if I can do it tomorrow in my engines class.) I have tested the Air Regulator bad, and pin 34 on both my ECU, and the big plug look burnt, in fact on the ECU it looks like theres a solder(sp)(SAW-DER) at the bottom of the spade terminal. I am pretty sure this is what is making it run lean. and How would I go about fixing (if even needed) the terminals? I only did a continuity test, through my automotive electrical class I was taught mainly voltage drop, cuz apparently even if theres 1 strand of wire going the sensor will still read continuity? idk but it tested bad so it's gotta be bad lol

 

For the burnt terminals should I be looking at too much voltage or ? I can take pictures if needed.

Thanks!

Edited April 8, 2010 by Phillip

[19762802+2]

Have you ever cleaned out you gas tank or replaced you fuel filters? If not you should probably do that soon because my 75's Injectors when I bought the car went out and when I took them off I noticed little metal flakes inside on a small filter in the injectors. All of this was because of a rusty old gas tank which is common for Z's. So you may want to drain your tank to see what the fuel inside is like if it is red or if there are small chunks of metal in it.

 

Good luck with your project though