Page updated on 06-19-2018

96 GMC Jimmy black smoke, backfires, Timing?

01-11-2010, 06:38 PM
Any help is much appredciated,
I have a 96 GMC Jimmy, it is hard to start. It has black smoke out tail pipe. It backfires. It has a bad smell of gas out the tail pipe. It runs horribly. The computer only reads one code, Multiple cylinder Missfire. and also gives me data. That My IGNITION ADVANCE IS: - 24, What does that mean? is my timing off? the Jimmy has 240,000 miles, i would suspect that the timing chain might be worn. I have replaced the following, The fuel regulator, and cleaned the spider. I replaced the plugs, wires, cap , rotor, new map sensor, new maf sensor. I cleaned the EGR valve. I believe that the computer has also put the transmsiion into "limp home mode" because it had all the gears but now, is only in second. Does anyone know as well, on the ECU there are three plugs that should be plugged in or two. Mine has a place for three plugs. and the center one was unpluged i just plugged it in, didnt change anhything????? ANyways ANBODY HAVE ANY IDEAS ??? Thanks SO MUCH Cory-:licka:

01-11-2010, 06:44 PM
1) Do a smoke test on the EGR, PCV, and Intake.
Do a google ''Youtube Auto smoke test''

01-11-2010, 06:46 PM
I know that the Jimmy is running too rich, i can smell the gas...

01-11-2010, 07:50 PM
2) Do a compression test next
3) What about a upstream H02 sensor for the air/fuel mix.

01-11-2010, 08:19 PM
I can do a compression test, i do not think that is the problem though. I have not got a code for the oxygen sensor, i beleive it would set one off.

01-11-2010, 08:21 PM
I am going to pull the timing cover off tommorow. If it was not the problem at least i will have eliminated one variable.:)

01-11-2010, 10:21 PM
You need to loosen the oil pan to remove the timing cover, and the shop manual recommends replacing the timing cover if removed to avoid oil leaks. Quick easy way to tell if the timing chain jumped, (as long as nothing has been taken apart since it started acting up).... Remove distributor cap, rotate the crankshaft manually with a socket and ratchet until the distributor rotor lines up with the "6" on the distributor housing. Then make sure the timing mark on the damper is lined up with the mark on the timing cover.

01-11-2010, 10:34 PM
Where is the mark on the damper... And where is the Damper? I have not worked on a timing chain before. So i look to make sure these to lines line up with the crankshaft at TDC ??Thanks for the help..

01-11-2010, 10:52 PM
I just thought of something, since the reading i got for the ignition timing was -24 could that be the reading for the camshaft retart? should i try replacingh the camshaft positioning sensor???? :runaround:

01-11-2010, 11:08 PM
The damper is the large pulley on the crankshaft that the belt goes around. The mark is just a groove in the damper. There are two grooves but only one will be close to the mark on the timing cover. There is a tab with line on it on the timing cover and the two marks must line up. As you face the timing cover, the tab is approximately at the 2:00 position. The -24 degrees is the amount of retard the ECM is commanding. When a timing chain jumps time, valve timing retards along with ignition timing. The -24 degrees along with a lack of power might be an indication the chain has jumped and the ECM is tryng to compensate for it. Check the marks as mentioned, before replacing parts needlessly.

01-11-2010, 11:12 PM
Thanks So Much !!
By turning the rotor to TDC do i need to turn it back to where it was after this test??

01-12-2010, 02:47 PM
Nope, it turns when the engine is running. You're just putting it a "given" position so you can check the syncronization of the camshaft and crankshaft.

01-12-2010, 03:25 PM
Ok, i did the test on the timing, And the rotor lined up perfectly with the 6.
So, i did not jump time?
After doing the test i tried starting it again, it just backfired throught the throtle body... I can get it to start and run for like a minute then it die out and backfires like the whole time it is running.
I amd wondering now if maybe the ECU computer is just FRIED or something.?????:confused:

01-12-2010, 04:05 PM
If the rotor was on the "6" and the damper lined up with the tab on the timing cover, (at the same time ;) ) the chain has not jumped.

Have you checked fuel pressure and leakdown?
What brand spark plugs and how old are they?
What brand and how old is the distributor cap & rotor?
What brand and how old are the plug wires?

01-12-2010, 04:53 PM
I used Bosh platinum plugs. I used Duralast cap and rotor.
I beleive they where duralast wires as well. I just put them on last week. And i put the new fuel regulator on last week too, and i cleaned the spider and poppet injectors . i put a new map sensor on it, and got a used mass air flow. I also cleaned the EGR valve with carb cleaner.. It also has a new fuel pump last year. Am i looking for low or too much fuel pressure. As it is backfiring and the fuel smell i assumed it was getting to much fuel,, or not neough air, spark??????
Thanks So much again !

01-12-2010, 05:13 PM
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. ;)

01-12-2010, 05:21 PM
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

01-12-2010, 05:32 PM
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?

01-12-2010, 05:47 PM
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

01-12-2010, 05:52 PM
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.

01-12-2010, 06:05 PM
is there a way to test the coil? And would the jimmy still start if it is faulty?

01-12-2010, 06:27 PM
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.

01-12-2010, 09:21 PM
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.

01-13-2010, 07:26 PM
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..

01-13-2010, 07:37 PM
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.

01-13-2010, 08:39 PM
I have not go to work on th Jimmy today, got sick.... The knock sensor?? what is its function? And where is it located???

01-17-2010, 09:09 PM
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.:smile:

01-17-2010, 09:36 PM
Check fuel pressure and leakdown, and post the results.

01-19-2010, 01:54 PM
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.:)

01-19-2010, 02:36 PM
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.

01-19-2010, 02:59 PM
I still have the Duralast cap and rotor and wires, I did Replace the Plugs to AcDelco. I do believe that did help it a bit.. I can try putting AcDelco cap rotor and wires... Would that really make that big of a difference? I have never had trouble with buying that brand before on other vehicles. I will have to ask about what the specs where on the Fuel System.. Thanks

01-19-2010, 06:03 PM
There are several threads on AF where members have installed Duralast secondary ignition parts on their 4.3 engines, (do a search). They have experienced problems either right away or shortly after the parts were installed. This includes all GM vehicles with the 4.3 "W" & "X" engines, not just Blazer & Jimmy. If you were to put the vehicle on a scope, you'd see immediately if the problem was caused by a fault in secondary ignition. But, since a scope is not a piece of equipment found in the average DIYer's garage, it's another important reason to use quality parts. I'm not saying your parts are faulty, but it's a possibility. I would suggest starting with an AC Delco cap and rotor, and see if you can get the fuel pressure and leakdown results. Might not be a bad idea to add a fuel pressure gauge to your tool box ;) They're like $20

01-21-2010, 12:04 AM
I reinstalled the EGR valve today.I also disconnected the battery to reset the computer. And just for the hell of it i started the jimmy in the driveway and let it run for about 15 min. It actually stayed running the whole time. But it began idleing higher than normal. Anyways...I plugged in the OBD2 Scanner And i got new Codes ! ! they are
-P0300 multiple cylinder misfire
-p0134 O2 Sensor circuit No activity detected
-P0146 O2 sensor circuit no activity detected
-P0152 O2 sensor circuit High Voltage
-P0753 Shift Solenoid "A" Electrical
So, has the computer detected a bad O2 sensor?? and how can i tell wich one, i think there are Four on the jimmy
Thanks, :smile:

01-21-2010, 11:51 AM
DTC's must to diagnosed and repaired in the order that they appear. Then after the repair, clear all DTC's from the memory, and perform a drive cycle to confirm the repair. Disconnecting the battery will turn the SES light off provided there are no hard faults. It will not remove the DTC's from memory, that must be done with a scan tool.

Whatever caused the P0300 to set, likely caused the other DTC's to set also. After diagnosing, repairing the cause of the P0300, clearing the memory, and performing a drive cycle, it's quite possible the other DTC's will not return.

P0300 is almost always caused by a fault in the secondary ignition system: Spark plugs, spark plug wires, distributor cap & rotor, and ignition coil however, it can also be caused by low fuel pressure.

01-21-2010, 12:23 PM
I am sorry but u have no resaon to believe i have a fuel pressure problem. I have Replaced the Fuel pump, the Fuel filter, And i just replaced the Fuel regulator, and the "Spider". And my mechanic already looked at my fuel system... My battery had a drain on it untill three days ago. This was causing the computer to continusily be reset.. Therefore it could never save codes... And Yesterday was the first time since i owned the Jimmy that the truck actually ran long enough to heat up.. I believe that the vehicle must warm up in order for the oxygen sensor to have set a DTC. I Have also replaced every single part in my secondary ignition system.. Even the ECU ! coil, ignition module,.So i Am eliminating that as an option as well. So that leaving me to the last option.. being a Fault in my Emissions system, and i have already cleaned my egr valve, bought a new map sensor and maf sensor. And the fact that i do have DTC's for Oxygen sensors.. I am going to buy the oxygens sensors.. TO be honest .. I have replaced everything else that could be causing this problem.. its change these are its goin to the junkyard. I have listed all the parts i replaced in all my messages i posted. And i found the sensors for 20 bucks each on ebay. Thanks for the help

01-21-2010, 12:58 PM
You've replaced a ton of parts and spent a ton of money, and the problem is still there. Now would be the time to diagnose rather than guess. I am assuming that since you used a Duralast distributor cap, you most likely used an Airtex fuel pump, (Autozone sells both of those). The reason I suggest checking fuel pressure and leakdown is that Airtex fuel pumps are known to have low fuel pressure and leaky check valves.... right out of the box. Just because a part is new, is no guarantee that it's good.

01-29-2010, 12:06 AM
I got my fuel pump from Auto Value...
I Fixed the problem yesterday! My friend suggested because of the backfire to check my Firing order... Turned out two of my spark plug wires needed switched around. I started it up, and it runs Excellent. At least i know i have a ton of new parts...:iceslolan
Thanks for the help

01-31-2010, 02:27 PM
hahahahahahahahahaha i'm sorry but all that for a firing order! common always start at the basics! Nice try though old_master

01-31-2010, 03:29 PM
....I Fixed the problem yesterday! My friend suggested because of the backfire to check my Firing order... Turned out two of my spark plug wires needed switched around. I started it up, and it runs Excellent...

In your very first post you mentioned that you replaced the cap and wires... Let me guess, you switched #1 and #3 in the cap? It's a common mistake and also the reason to replace wires one at a time ;) Anyway, glad you got it going.

Add your comment to this topic!