Help with Erratic Gauges and Backfire

[nero6]

So, this weekend I went to a car show which is about an hour and a hlaf away. On the way down the car ran great. I parked it at the car show and it sat in the hot weather for about 8 hours. Started her back up to head back home and had no issues until a few miles down the road. First issue was the car was backfiring when downshifting much more than usual. It always backfired before but not nearly as loud or as often. I have a performance cam, an no catalytic converters. 2.5 inch exhaust with pretty free flowing mufflers, so I expect some minimal backfire or crackle in the exhaust. That was the first issue.



The second issue was when I was heading to the gas station to fuel up. I was waiting to take a left into the gas station when all the gauges went haywire. The volts dropped, the temp gauge shot up as high as it could go, and the oil pressure gauge dropped, but the car still idled fine. So I pulled into the gas station, as soon as I hit the throttle, everything came back to 'normal'. For the remainder of the drive in town, the temp gauge floated erratically, and the voltaged dropped at each stop. Once I got on the highway everything was fine, until I slowed down to take an exit when I was alomost home, and again the temp gauge floated up and down, and the volts dropped. I check the battery voltage when I got home and it was 12.85 so it seem to be charging fine. Anybody have any ideas on the backfire issue and the erratic gauges?

 

[mc0311grunt]

your volts seem kind of low. My 86 did the same thing when my alternator went out. Check the alternator. If you have the stock gauges then the would act nuts with power coming in and out.time to convert to an sn95 130 amp

 

[nero6]

your volts seem kind of low. My 86 did the same thing when my alternator went out. Check the alternator. If you have the stock gauges then the would act nuts with power coming in and out.time to convert to an sn95 130 amp

Ya, I was thinking that may be part of the issue as it happens under a load and at idle. I have a digital voltmeter but not sure how to test the alternator under load with it. I have to take it on a long trip in a few days so I am just going to drop it off at a local garage here to have it checked over. It will probably take me much longer than a few days to figure it out, and I do not want to end up stranded on the side of the highway. Hopefully it does not end up costing too much.

 

[ID89GT]

have the car running, and turn on literally everything electrical on the car, this will load the alternator and you can see what the volts are at then, they should stay about I think its 13.4 or something.. I don't remember for sure..

 

[TenorPlayinGuy]

should be between 13.5 and 14.5 when running. (if its stock alternator, run it at 1500rpm) We mean everything on. Lights (high beams) air (a/c and MAX) if equipped, radio, etc. My gauges did that once, but I think it was my fault because of a wire. A ground is what it sounds like, but I couldn't be sure unless I took the cluster apart and started testing.

 

[jrichker]

The secondary power ground is between the back of the
intake manifold and the driver's side firewall. It is often missing or
loose. It supplies ground for the alternator, A/C compressor
clutch and other electrical accessories such as the gauges.


Any car that has a 3G or high output current alternator needs
a 4 gauge ground wire running from the block to the chassis
ground where the battery pigtail ground connects.
The 3G has a 130 amp capacity, so you wire the power side
with 4 gauge wire. It stands to reason that the ground side
handles just a much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery & extra 3G
alternator ground wire as described above in paragraph 2. A screwdriver
points to the bolt that is the common ground point.
The battery common ground is a 10 gauge pigtail with the computer ground
attached to it.

Picture courtesy timewarped1972

The computer has its own dedicated power ground that comes off the ground pigtail
on the battery ground wire. Due to it's proximity to the battery, it may become
corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.

 

[nero6]

Thanks, I just dropped it off to the mechanic today, with a list of other small issues as well (car will not idle below 1100 rpm or it will stall, the backfire issue, and a few others). Oh joy Thankfully I do know him and I hope he will be able to figure it out and not charge me too much. It is way too far over my head to figure out in a few days. There are several wiring 'gremlins' that I failed to mention, so it could be just about anything right now.

One is that the ground wire that goes into the diognostic terminal in the engine bay is cut, and then a wire spliced in and grounded back to the negative battery terminal. I can still get the codes but instead of grounding the self test input to the diognostic terminal I have to ground it to the negative battery terminal. So there is a ground issue somewhere. There is also still a alarm module wired in the car, it is no longer active (as far as I know) but there are multiple wires spliced together and some open ended wires that I have since taped off. A big mess. It never affected anything before and everything worked fine, but that is probably part of the issue as well. I discovered all this 'wiring' shortly after I bought the car and just hoped that it would not end up causing problems later on. I guess I will know in a few days what the outcome is.

 

[nero6]

Several days later, 4 bad grounds, a new computer and I am finally back on the road. Still a few small bugs to work out but all the backfiring was due to the bad computer. Once that was installed, no more backfireing. The guage problem was solved by a bad ground along with 3 other bad grounds causing other issues. For an engine that was rebiuilt only a few thousand km ago they failed to fix any of the electrical issues.

Oh the joys of previous owners, from the prior owner using gear oil in the transmission (had to purchase 'new to me used t5 transmission', fog lights not wired (an lx switch was used and the fog light wires were taped off), ground on the right fog light was snapped off as well, the signal gound for pulling the codes in the engine bay was grounded to the negative battey terminal as the computer ground was toast, floor pans were patched with new metal but never painted so had to clean off a bunch of surface rust and paint, pulled out about a meter of useless stereo wire connected to nothing, still have an inactive alarm moduale wired, bad map sensor amoung a list of other issues that were fixed.....Oh well at least it is back on the road now.