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'87 MJ 4.0L backfire...gradually worsening

0

043500

NAXJA Forum User
Location
Ocala, FL USA
About 2 weeks or so ago I replaced a bad O2 sensor as well as the battery and starter cables.
After that I started hearing a faint but noticable backfire while cruising...not during acceleration or idle.
It became gradually louder and then began to do it under acceleration. Now it does it almost constantly except at idle and will do it if I give it gas while in park or neutral as well (not under a load). THere's not real pattern to it...it does it whether the engine is warmed up or not...ambient temp has no effect either...I've drivien it at 45* and 85* during the last couple weeks.

I replaced the fuel filter just for giggles (which did not fix it).

Of course I just gave my friend his scanner back after dxing and 'fixing' a tranny issue so I will be borrowing it again.

I am leaning towards ignition issues...?
 
What "scanner" do you have for a RENIX?

Anyway, sounds like you have a timing issue not a fuel issue. This problem could be mechanical or electrical.
 
Backfiring is many times a fuel imbalance issue.

I always start with a simple fuel pressure test. It must be ruled in or out early in the troubleshooting process.

For your 87, you should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Also, be darn sure that the vacuum line leading to that fuel pressure regulator is pulling good vacuum.

If you don't have access to a gauge, some big box parts stores that rent tools will rent you one inexpensively, screw on the proper adapter, screw it onto the schrader valve on the fuel rail and you will know your fuel pressure.

Also, good time to review all tuneup hardware when you have symptoms. Fresh Champion copper plugs gapped to .035, new plug wires, distributor cap and rotor and fuel filter.
 
It's a snapon mt-2500 scanner that I now need to re-borrow from my friend.

I do plan to do the usual tuneup items this weekend.

The MAP sensor was a little off (can't remember which direction) so I put in a spare that I had and did a battery disco etc. It was still doing it BUT was better.

I do plan on a fuel pressure test as soon as I can and will probably replace the coil soon too.
 
043500 said:
It's a snapon mt-2500 scanner that I now need to re-borrow from my friend.
Click to expand...

What he means is that there is no 'scanner' for a RENIX, as their computers do not spit out codes; the technology is too primitive. Only OBD-I and OBD-II will give codes out with a scanner. OBD-I started in '91, so you don't need to re-borrow it from your friend as it will be useless.
 
bdahlg said:
What he means is that there is no 'scanner' for a RENIX, as their computers do not spit out codes; the technology is too primitive. Only OBD-I and OBD-II will give codes out with a scanner. OBD-I started in '91, so you don't need to re-borrow it from your friend as it will be useless.
Click to expand...

Correct, what you need is the RENIX FSM so that you can test all of the RENIX inputs/outputs to make sure everything is in range.

I do not think(I've been wrong before!) that the Snap On 2500 will read the RENIX but its possible.

This isn't the RENIX FSM but it has some great RENIX info in it from one of our local's

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm
 
Well for you scanner doubters it does indeed read the voltages from the sensors. YOu can drive down the road with it and see it all happen in real time. It even has a transmission section and you can watch the solenoids turn on and off, convertor lockup, trans output RPM etc. Again you can see it all happen in real time...and while it won't necessarily throw out a CODE... it does for exampe: TPS voltage and you can see if it is in the range it should be...if not...bad or out of adjustment TPS...and it does this with the other sensors and sending uints as well. I just recently (as my originaly post showed) dx'd and replaced a bad O2 sensor. THen I was dumb and gave the scanner back before driving with it some more to see if anything else popped up.

Every renix I've owned or even seen has a port to plug this in with the proper adapter and card.

So believe it or not...there is a scanner for renix. Maybe not in the sense of the OBDI or II scanners BUT it is damn useful for this system. I do like to ask here as well as I am sure the scanner doesn't do it all so info and advice is useful.
 
bdahlg said:
What he means is that there is no 'scanner' for a RENIX, as their computers do not spit out codes; the technology is too primitive. Only OBD-I and OBD-II will give codes out with a scanner. OBD-I started in '91, so you don't need to re-borrow it from your friend as it will be useless.
Click to expand...

The Snapon MT2500 does read Renix, as did the AMC/Chrysler DRB scanners (which are rare).
 
043500 said:
Well for you scanner doubters it does indeed read the voltages from the sensors. YOu can drive down the road with it and see it all happen in real time. It even has a transmission section and you can watch the solenoids turn on and off, convertor lockup, trans output RPM etc. Again you can see it all happen in real time...and while it won't necessarily throw out a CODE... it does for exampe: TPS voltage and you can see if it is in the range it should be...if not...bad or out of adjustment TPS...and it does this with the other sensors and sending uints as well. I just recently (as my originaly post showed) dx'd and replaced a bad O2 sensor. THen I was dumb and gave the scanner back before driving with it some more to see if anything else popped up.

Every renix I've owned or even seen has a port to plug this in with the proper adapter and card.

So believe it or not...there is a scanner for renix. Maybe not in the sense of the OBDI or II scanners BUT it is damn useful for this system. I do like to ask here as well as I am sure the scanner doesn't do it all so info and advice is useful.
Click to expand...

Have you read my old thread:

http://www.naxja.org/forum/showthread.php?t=905849

Have you checked for 12-14 volts on the orange wire at the O2 sensor? Have you checked for a wandering resistance sensor ground, which is common with Renix? Did you use a Bosch OEM style 3 wire O2 sensor? Have you check for a vacuum leak anywhere near the vacuum line to the MAP sensor!!!!! Also might check the real time read out of the CPS, to see if it is about to die!

Usual suspects are the TPS, O2 sensor circuit, and vacuum problem with MAP sensor that cause missfires.
 
Last edited:
toates89 said:
my renix backfired turned out to be a tare in the FPR
Click to expand...

That is why I suggested fuel pressure testing. It is the FIRST thing I do with any vehicle that is backfiring. Takes all of ........10 minutes?

Not saying that is what it is. But I am saying that it needs to be verified. Any shop tech. would agree.
 
Ecomike said:
Have you read my old thread:

http://www.naxja.org/forum/showthread.php?t=905849

Have you checked for 12-14 volts on the orange wire at the O2 sensor? Have you checked for a wandering resistance sensor ground, which is common with Renix? Did you use a Bosch OEM style 3 wire O2 sensor? Have you check for a vacuum leak anywhere near the vacuum line to the MAP sensor!!!!! Also might check the real time read out of the CPS, to see if it is about to die!

Usual suspects are the TPS, O2 sensor circuit, and vacuum problem with MAP sensor that cause missfires.
Click to expand...

I have not but will. I have checked the MAP vac line and actually swapped in another sensor.
The other stuff is planned to be done asap. THe readouts on all the sensors (checked last week) were all within their ranges. but since it is worsening they will eb checked again.

birchlake...a fuel pressure test is at the top of the list. The other things I mentioned I have done were things I could do right then and there...I don't have the tools to do the test and have not had a chance to borrow/buy them yet.
 
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