Car runs fine one day next day backfiring?

It could be many things, start fresh by disconnecting the battery for atleast 15 mins, then restart it and let it run. Make sure you turn on the heat so it can use to the load. Double check timing and fuel pressure.... Then see if it starts popping again. With all the idling that is the only load the car has really seen, it needs to be driven once all vitals have been checked. A bent valley usually pops badly all the time, same goes for too tight of a lash. Did it rev good before the pop, for instance when you seated the rings?

Does the car have a open breather or a closed stlye like stock with the vac tube going to the TB? Aftermarket or stock MAF? What is the TPS voltage set to?

What is the exact combo? I assumed this was a brand new motor?

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 19-May-2009 to update drawing for dumping the codes on 86-88 Mustangs with no check engine light.

Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical





If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.

Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

NewToFord said:

I went to check timing today and my timing light was broke. SO I have to get a new one. The car wasnt backfiring today. It did a few times but then it was ok for the most part. One thing I did notice is that the pcv valve had popped out of the intake. I guess if it is not backfiring all the time then the valves and rockers are ok. I did have some kind of light whistle noise at idle, but as soon as you rev the car up it disappears, so I have to look into that.

Click to expand...

what tb? bbk/edel?

If the PCV valve is out or loose in its rubber gromment, that is an unmetered air entry point. That will do the same thing as a vacuum leak - lean mixture and backfire.

the stupid grommet on my pcv is a pain in the arse as it seems to always be loose. I even bought a new grommet and it doesnt fit tight enough in the lower intake. Now that I think of it, maybe I should put some type of rtv in there to hold it