97 DX electrical problem

[Keitho97DX]

My project 97 DX has been acting up lately. I am blowing the #15 7.5 amp fuse under the dash when 1.6L motor comes up to temp. That circuit contains the alt, both o2s, vss and who knows what else. 1st the check engine code told me prim o2s. so i replaced it. Returned after a month. I isolated components by pulling the connectors. I thought alt was the cause. Replaced it & you guessed it its back. code is saying 02 heater, o2s circuit fault? Even if all the connectors are pulled, it pops the fuse. It won't pop the fuse until I bring the rpms up a little with the motor at temp. HELP a frustrated newbie out. Any suggestions would be appreciated?

[imported_DIYperformance]

That's actually a problem many 6th gen owners run into. The issue is that the one fuse acts for various important components such as your alternator, vss (vehicle speed sensor), O2 sensor, TCM, and some other stuff too.

It's going to come down to one of two things, either one of the components is not working or you have a short. To figure it out, put a good fuse in and turn the ignition to "ON" but don't start the car. The fuse should NOT blow at this time. Hook up a voltage meter and test it at the fuse. If the power reading is surging and the car is only at the "ON" position, then you have a short. Most of the time it's coming from the alternator but don't rule anything out. If the power reading is not surging, then test the components that are linked to that fuse.

I also had this same problem. Mine was a short in a wire. I don't remember exactly what, because it happened right after I got the car like 6 years ago.

[Keitho97DX]

Thanks for the test tip. I'll give it a try. I hate electrical issues that are intermittent.

[K5Calamity]

I had an idea I was going to put, but I was headed the wrong direction. The other items fuse #15 feeds aside from what you guys already mentioned are the purge control solenoid valve, bypass solenoid valve, and canister vent shut off valve.

[imported_DIYperformance]

^yeah I wasn't sure of the specific pieces aside from the major components, but thanks for the technical input!

[K5Calamity]

No problem, I did some more studying and found out that at 154 deg F engine coolant temp, the ECM takes control of the EVAP purge system, which has to do with those solenoids I mentioned above. I think the purge control solenoid valve has an internal short which is blowing #15fuse when the ECM is turning it on to purge the EVAP canister of fuel vapors. Makes sense because you also mentioned it blows when you raise the RPM. The ECM senses that and with the temp data, TPS position and manifold absolute pressure (MAP) it commands the valve to purge the canister.

Try unplugging the purge control solenoid valve and see if the fuse still blows. If that isn't it unplug the solenoid valve on top of the EVAP cansister. If that wasn't it, unplug the solenoid valve on the fuel tank. If it still blows after doing all that, then I wasted about 10 minutes of your time. But now I'm outta other ideas.

Good luck!!!

[Keitho97DX]

Thank you for all your insight. Once I get a chance to play and possibly fix, I will let you know how it turns out! Hard to work on car when the high temp for the day is -5°F :-(

[Keitho97DX]

After reveiwing your input, I realised something strange that happens a lot. It might apply, or not. When the fuel tank is near empty the car cranks a lot more to start than with a full tank. Also when I remove the fuel cap on an empty tank, air pressure blows past the cap. I replaced the cap already. Could this be related? Thanks

[K5Calamity]

Yes, taking longer to start can be related to the EVAP system and alot of other things as well. Only systematic troubleshooting will lend a good repair. However, perhaps I can offer something useful here that will help. This is going to be long and drawn out, but I made every effort to sound interesting.

EVAP System Overview: When nearly empty, the fuel tank fills with fuel vapors and pressure rises. (the temp of the fuel rises from circulation through the system and ambient outside temp as well, if its warmer than the fuel temp).

The EVAP system is designed to automatically dump the vapors and relieve the pressure inside the tank when it reaches a set pressure value, or during engine start when the ignition key is turned to the on position.

Pressure in the tank is monitored and dumped automatically as needed. There is a pressure sensor and solenoid valve on the tank that does this. They are tied into the eng computer.

When the pressure and vapors in the tank are dumped they are routed to the EVAP canister mounted on the firewall for temp storage. From there they are drawn into the eng intake manifold to be burned in the eng.

The purge control solenoid valve I talked about earlier dumps the EVAP canister when commanded by the eng computer. Remember the eng coolant temp must be at or above 154 deg F. The system is inhibited below that temp. Depending on your car there can be other factors involved as well, like whether you are moving or not, but don't worry about that now.

Ok, that gives a basic system overview. Now as far as the taking longer to start when tank is empty, here goes. My 1st thought is a vacume leak through the purge control solenoid valve. You can apply vacume to the EVAP canister and purge control valve each individually with a hand operated vacume pump to check for leaks.

A vacume leak would result in lean condition, which would result in delayed start, till the computer made the appropriate adjustments from baseline. I also suspect that solenoid valve is the source of your short circuit.

Well thats enough armchair troubleshooting for now. Let us know what happens in your journey.

[Keitho97DX]

Wow, you know your stuff. One last question. Since I have no experience on this. How do I access the Purge Control Sol.? I'm not sure if you mentioned its location. Thanks again for your help.