Howdy! This is my first real post on the Forum after my introduction.

Back story: Alright, so I have a 1999 XJ with a 4.0. I bought the Jeep on Marketplace as a project, and it came with a parts Jeep. The guy I bought it from got it as a rolling chassis and swaped the engine and tranny out of the Parts Jeep into it.

The jeep has a number of issues, but the main one that I have been working to solve over the last few months is a backfire out the intake when revved, along with a hard start, rough running, not idling, and the inability to rev over ~3,500 RPM. Then when you shut it off, the fuel injectors continue to fire sporadically for about 30 seconds. The PO told me that the engine ran in this condition before he swapped it out of the parts jeep. He already changed the CPS with a cheap one off of amazon, so I suspected the Crankshaft position sensor was still the problem. He also installed 4 hole injectors, a cheap replacement Throttle body with pre-installed TPS MAP IAC, new distributor, cap, rotor, plugs, wires, and the fuel pump appears to have been replaced as someone has cut an access pannel out of the rear cargo area to get to the pump.

So I plugged in my scanner to read all the codes. It had several relevant codes including the Crankshaft position sensor, TPS voltage low, IAC valve, No engine Bus, and an o2 sensor. I replaced the IAC which allowed the engine to start and idle easily. Then I temporarily swapped a known good crank sensor out of my buddies running XJ for testing purposes, and it changed nothing. That stumped me. Fuel pressure is good at the fuel rail, Crank sensor Pigtail has 5 volts at power and reference pins and the the ground has low (<0.2 ohms) resistance all the way through the harness and pcm to the battery. The guage cluster DOES work, and I also swapped the PCM out of the parts Jeep but it changed nothing. After a few months, the CPS code no longer shows up but the jeep continues to behave the same way. I went through and repaired a bunch of grounds coming out of the wiring harness, and I found that the ground strap going from the back of the head to the firewall was missing. Immediately After fixing that, the jeep ran better and stopped backfiring through the intake. The MIL turned off, no more codes were present, but the jeep still wasn't running right and wouldn't rev up well.... This lasted for about 20 min, and then it began the backfire again. Im running out of ideas, so I decided it was time to get some help. Any advice for further testing, previous experience, questions, anything is appreciated! Thanks!

 

Chasing your tail looking at the cps, it'll either run or it won't.
i would pop the distributor cap and check mechanical timing if someone has been playing with the distributor. Then look at data streams on your scanner and see if anything looks goofy. Maybe tps or map?

 

Ok. Well, I have heard that many of those cheap CPSs will still run but will wreak havoc on things because the pcm doesn't like them. But I may be wrong. Also, I had considered a Mechanical valve/ignition timing issue but the backfire is not consistent. It is worse at times, and others it completely goes away which makes my thoughts go more towards electronics. I forgot to mention that I did swap the MAP sensor for a known good one so that's ruled out. I probed the back of the TPS connector and checked the voltage while opening the throttle. I don't have an analog volt meter (which is better to be able to see any dead spots or skips/jumps), so I just used a digital multimeter. The sensor tested fine from what I could see

 

firing out the intake is usually a sign of crossed dist wires, somehow a completely screwed advance or retard, or super lean. A compression test will show if previously someone mistimed the cam. Do you have a timing light? hook up to #1 wire and see if you are close. normally advance-retard is totally computer driven. Does it run well or long enough to hook ap a simple scanner/code reader with data display...should show around 9-10 advance in idle . I find it highly unlikely that the injectors are firing after shutdown. Buy a noid light set...cheap. That will tell you

 

I did check, and all the distributor wires are attached properly. I do not have a timing light, but I'm also not sure how you would check the timing because there are no timing marks on the harmonic balancer. Maby im missing something. I have yet to do a compression test, so I will go ahead and do that probably this weekend. The Jeep starts right up and will idle pretty smoothly for as long as you want it to, so I can check on the timing advance. Im almost positive its the injectors that are continuing to fire, but I should be able to verify that by shutting the engine off after running, and quickly unplugging all the injectors. The clicking always last for at least 30-60 sec so that should be enough time. Ill do those couple things, and then update.

 

The fuel injectors are all powered through the asd relay. They should be dead when key is turned to off

 

Okay, so I checked the timing advance for #1 cylinder at idle-3500 RPM. The timing Advance is high at idle. I attached a link to a video of the live data. I also pulled the distributor cap off and checked for play in the distributor shaft. The shaft does have quite a bit of play. I also attached a video showing the play that the shaft has. I noticed that the inside of the distributor cap has scratches around the plastic on the inside that appeared be from the rotor. I'm not exactly sure what to do with this information. (Im sorry in advance, you may have to copy/paste the links. I know there is a way to make them open from this site, but I don't know how)

Timing Advance Live Data:

Play in Distributor Shaft:

 

Yes that play is excessive. I would be looking for a new distributor. Not sure what to tell you on the timing, thats over my head.
IMO the scratches may be from soneone cleaning the contacts with a large wire brush.

 

Your advanced timing might be due to the engine at almost 1500 rpm during your video. The timing numbers i gave is for 650-700 idle. I don't know why your at 1500. The timing advance goes up with rpm increase I don't see how the rotor could have caused those scratches. At first I was expecting rotational play in your rotor....not lateral. Yes you need a new dizzy. Quite some time ago I won one on ebay for a dollar plus shipping. I might have my old 92 in a box in the attic but don't t think the connectors are interchangeable across those years

 

Im currently running a 98 stroked in my 2001 so I use a cam syncronizer instead of the dizzy it came with. Its been almost 10 years since the rebuild and I can't remember if it came with the dizzy. I can check later but it might not be till next weekend will let you know

 

In that video I sent of the timing advance, the jeep was Idling around 730 rpm for the first half of the video, not 1500. Then I revved it up to 3500. The advanced timing you see was at normal idle.

So I definitely need a new distributor, but could that be the cause of the timing advance and ignition timing issue with it simply having play in it? I still think there is probably something else going on in addition to the distributor.

 

Ok, so I've done some further troubleshooting. I did replace the distributor, and found that the gear on the old distributor shaft was installed 180 off which put the rotor between 1 and 2 with cylinder 1 on compression stroke TDC. So that was one timing issue that I found and fixed, but did not solve the backfiring. I was watching live data and noticed that my short term fuel trim was holding between -14% and -18%. This means its pulling that percentage of fuel from the mixture essentially leaning the mixture. This would happen if the o2 sensor read a rich condition ie. too much fuel in the mixture. However, there are no rich codes, or anything to support that. I have noticed some white deposits on spark plugs, and combustion temps seem to by getting high, as the exhaust manifold begins to glow hot after idling for a while. In my mind this would point to a lean condition. So my theory was that it was a "false Rich condition" caused by a single leaking injector that was dumping fuel in. The computer was compensating by pulling fuel from the mixture creating an overall lean condition. To further support my theory, fuel pressure would drop off quickly as soon as the engine was off, and after sitting for a while, it would only run on 5 cylinders possibly untill the flooded cylinder dried up enough to fire. So I threw an old set of injectors in it that I had and it seems to hold fuel pressure at the rail for a while after the key is shut off now. But it still runs terrible and backfires out the intake. Its not a steady backfire so I doubt it is a mechanical timing issue at this point. Where do I look next?

 

Is your intake manifold on correctly? Common issue

 

Well, what do you mean by that? Are you referring to proper torque sequence? I did try checking for vacuum leaks with propane, but I couldn't find anything

 

Disconnect your O2 sensors...it will go into pre programmed fuel flow mode based on other parameters than your O2 sensors feedback. That programmed fuel mode runs very well, just not great for emissions. If backfire and glowing manifold still present...its not an O2 sensor issue.