96 GMC Jimmy black smoke, backfires, Timing?

[old_master]

Is the check engine light on when running?
If so, what DTC's (diagnostic trouble codes) are set?

in order of importance:
#1 Check fuel pressure and leakdown and post results
Ignition in run position, engine off, fuel pump running: Must be 60psi to 66psi and must remain above 55psi for 3 to 5 minutes after the fuel pump shuts off.

#2 Bosch plugs in the 4.3 is a known problem, use AC Delco 41-993 plugs

#3 Duralast cap rotor and wires, although new, are uh, lets just say AC Delco would be a MUCH better choice.

[corhey88]

i will test the fuel pressure, as soon as i ge ahodof a gauge.. I was wondering if the ECU has not went bad.. I just bought this jimmy, i got it in the condition it is in now, and i had to plug in the top middle plug back in the ECU computer... i wonder if that had anuthing to do with the way its running or possibly damaged the computer running it like that??
Also i think i have a fuel filter that fits the jimmy in the garage.. Would a plugged up filter cause it to run the way it is and backfire??? hard starting??
Oh,, And the engine codes i get are....P0300.. Random Multiple cylinder misfire

[old_master]

Those are all possibilities, some more likely than others. Diagnosis needs to be done in a certain order to avoid unnecessary repairs. Is the check engine light on?

[corhey88]

yes the check engine light is on, but only reads one code, P0300 multiple cylinder misfire....kinda leaves me clueless seeing i replaced alot of parts already... But when i first scanned it, it read codes for map sensor low input and mass air flow low input.. and i did get a code last week for transmission solenoid A
????

[old_master]

P0300 is almost ALWAYS caused by secondary ignition... plugs, wires, cap & rotor. A faulty ignition coil can cause it too, but that's extremely rare. Low fuel pressure and or excessive leakdown can also cause it.

[corhey88]

is there a way to test the coil? And would the jimmy still start if it is faulty?

[old_master]

Using a DVOM:
Resistance from coil wire connection to primary terminal should show 5k-25K ohms.
Resistance from primary terminal to mounting bracket should show infinite resistance.
Resistance between the two primary terminals should show .1 ohms.

If any are out of spec, replace the coil.

Keep in mind: just because all of the measurements indicate ok, does not necessarily mean the coil is good. There could be an intermittent condition with the coil when the vehicle is running and the coil is under a load.

Don't get too far off track here, take care of the "known" problems first.

[old_master]

Another thing you can try just for grins and giggles, is connect the scan tool, disconnect the knock sensor, start the engine and watch ignition timing and see if it changes from the -24 degrees you were showing. ...just a hunch that needs to be shot down

It will set a DTC for the knock sensor when you do this, just clear it out after you finish the test.

[corhey88]

I have not got to work on the Jimmy today..Got Sick...The Knock SENSOR? What is the function of that. And where is it located?
I am going to have to buy a haynes manual from the store friday..

[old_master]

The knock sensor is located next to the distributor and under the oil pressure sending unit, it screws into the block. It "listens" for spark knock and if detected, the ECM will retard ignition timing, up to 90 degrees, until the knock sensor no longer detects the knock. If the sensor fails, the ECM will set a DTC and the SES light will be illuminated. However, the sensor output could still be within range and yet eroneously indicating a knock is present. If that's the case, it could explain your 24 degree retard. Granted, it's an outside shot, but worth looking into. Save your money on the aftermarket manuals, they are far too general and many items are incorrect. If you want a manual, buy the GM factory shop manual. The GM website has them.

[corhey88]

I have not go to work on th Jimmy today, got sick.... The knock sensor?? what is its function? And where is it located???

[corhey88]

I have now changed the ECU computer,I got a used one at a junkyard for 40 bucks. And I changed the spark plugs to ACDelco, this did make it start again, the bosh plugs i pulled out where foiled out. I think that my problem is a Ignition sytem problem..... But, like i said before, i only get a P0300 code, wich is Multiple/Random cylinder misfire... SO, what ignition componant could go bad and not trip a code, and cause the Jimmy to run bad, stall, backfire....?????? Thanks so much for helping me.

[old_master]

Check fuel pressure and leakdown, and post the results.

[corhey88]

I had a mechanic friend look at my Jimmy, he says that the fuel system is working properly. Yesterday, I went and got a Ignition coil and Ignition Module. This did not change anything either.. I have changed plugs,wires,cap,rotor,coil,ECU,..I think i can eliminate a spark or ignition problem right? And i was told my Fuel System is fine. So Am i looking for a Emissions problem?? HAve a question though, while the Jimmy is running i unplugged the Egr, it did not effect the way it ran, and threw a Trouble Code. And am cleaning it now to put back on. Does that mean that Egr is Working? What is the the purpose of the Camshaft Sensor? and how would you go about figuring out if thats the problem??? i do still have the Ignition adavance (retart) at -22 ??? And the truck still backfires and stalls sometimes at idle. And Lack of power. It really does not hav the black smoke anymore. Just sometimes when it backfires.??? THANKS SO MUCH for the help.

[old_master]

At this point, you can't rule out anything because you have not diagnosed the problem.

The EGR valve is not supposed to be open at idle, so disconnecting it should not make a difference in the way it runs. Setting the DTC is normal, the ECM detected it was disconnected.

The camshaft position sensor is mainly used for detecting misfire. If it fails, it will set a DTC.

Do you still have the Duralast cap rotor and wires on it, or did you replace them when you replaced the plugs?

What is the actual fuel pressure and leakdown rate? Fuel delivery is a very common problem and is often tested incorrectly, misdiagnosed, and as a result, presumed OK.