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Discussion Starter · #1 ·
Working on a new YJ (1995 4cyl automatic) that wasn't running when I bought it. Previous owners said it ran but really rough, and it was due to the distributor being 180deg off.
I popped off the valve cover, set to TDC (double checking the #1 valves were correct), reinstalled the distributor, everything lined up.
Pumped out all the old fuel to be safe (using the fuel pump, so that works good), and put fresh fuel in. Also put in new spark plugs, new coils, new battery, new engine relays, new maxi fuses, and new fuses inside.

When I start it, it fires up and runs good for a few seconds, then starts cycling through running super rough (occasionally dying), and then back to really strong and smooth. Cycles every 10sec or so.

Never encountered this before, and I can only find a vague list of things to start checking, but I wanted to see if anyone has had this issue or knows a good place to start?

Thanks!
 

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Moderation in Moderation
1995 YJ Wrangler SE
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Was anything mentioned about the age of the timing chain?

I know you changed a lot of electrical, any wird on the age of the sensors? I assume this is happening when cold and hot... correct? The sensors might not matter at this stage of the trouble shoot.

Injectors clean? Pulsing properly?
 

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Typical /f12 Hooligan
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smooth then rough running in cycles could be a lot of things. we really need to narrow it down.

Easiest to hardest:

Check your YJ for trouble codes. It may be trying to tell you what's wrong . .

1. Vacuum leaks. Take some ether - starting fluid, and with the engine running, spray short bursts around the intake manifold, the vacuum lines, and listen to hear if the engine idles up. if it does, check the last area sprayed for broken tubes, bad vacuum connectors or loose manifold bolts.
2. Fuel Pressure - check Idle fuel pressure at the Schrader valve on your injector rail. If pressure is low, or fluctuates a lot, change fuel filter and or fuel pump.
3. Double check the ground wires on your coil. There are two posts on your coil mount that are grounds. If the bolts aren't tight, they can cause erratic idle. Ask me how I know.
3. IAS - Idle Air Sensor could be on the fritz. using a t20 torx, remove the AIS from the throttle body. Using THROTTLE BODY CLEANER (NOT Carb cleaner or brake parts cleaner) spray off the IAS and the housing it sits in. Reinstall and test. if it still occurs, it could be that the IAS is bad, its a $70 part to replace. Go to NAPA. Other store branded ones I've tried failed out of the box.
4. If this is happening at Idle, it may be your TPS Throttle Position Sensor. again, on the throttle body. run a test from the FSM or buy a new one. Helps if you have a sweep analog meter.. most don't...
5. Double check the fuel injectors. Use a set of NOID lights to test the firing of each injector. if one is failing, it could present as a misfire and cause rough idle.
 

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Discussion Starter · #5 ·
Was anything mentioned about the age of the timing chain?

I know you changed a lot of electrical, any wird on the age of the sensors? I assume this is happening when cold and hot... correct? The sensors might not matter at this stage of the trouble shoot.

Injectors clean? Pulsing properly?
Got the timing chain exposed today. Minimal slack and no delay in rotation between the two gears, so I think that's good (plus very new looking timing guide means it was probably replaced recently)

I was told previous owners replaced most sensors, but I'll take that with a grain of salt. Putting in an order for a few thing tonight.

Happening cold and hot. Injectors will be the next thing I'll inspect while I wait on parts.
 

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Discussion Starter · #8 ·
smooth then rough running in cycles could be a lot of things. we really need to narrow it down.

Easiest to hardest:

Check your YJ for trouble codes. It may be trying to tell you what's wrong . .

1. Vacuum leaks. Take some ether - starting fluid, and with the engine running, spray short bursts around the intake manifold, the vacuum lines, and listen to hear if the engine idles up. if it does, check the last area sprayed for broken tubes, bad vacuum connectors or loose manifold bolts.
2. Fuel Pressure - check Idle fuel pressure at the Schrader valve on your injector rail. If pressure is low, or fluctuates a lot, change fuel filter and or fuel pump.
3. Double check the ground wires on your coil. There are two posts on your coil mount that are grounds. If the bolts aren't tight, they can cause erratic idle. Ask me how I know.
3. IAS - Idle Air Sensor could be on the fritz. using a t20 torx, remove the AIS from the throttle body. Using THROTTLE BODY CLEANER (NOT Carb cleaner or brake parts cleaner) spray off the IAS and the housing it sits in. Reinstall and test. if it still occurs, it could be that the IAS is bad, its a $70 part to replace. Go to NAPA. Other store branded ones I've tried failed out of the box.
4. If this is happening at Idle, it may be your TPS Throttle Position Sensor. again, on the throttle body. run a test from the FSM or buy a new one. Helps if you have a sweep analog meter.. most don't...
5. Double check the fuel injectors. Use a set of NOID lights to test the firing of each injector. if one is failing, it could present as a misfire and cause rough idle.
Thank you! Lots of good things to check. And no check engine light/code, but I'm not entirely sure the light works.

1. Next on the list to check
2. Checked and flowing good and steady
3. and 4. on the way, weren't that expensive.
5. Also next to check
 

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Discussion Starter · #11 ·
You can turn up the idle enough to keep it running; the carb spray will still cause the rpm change.
Well, I had tried a few times when it was idling to push the pedal down a bit to try and get past the rough idle, but didnt change it, still choked and stuttered. I guess realizing that, I can probably rule out IAC.

I can probably rule out everything else today since I was going to check the PCM, and saw it labelled as a 4.0L, lol. Guess the previously owners didn't get one for the right engine when they were replacing it.
 

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Discussion Starter · #14 · (Edited)
Is that even possible ?
Edit: I decided to double check the pcm was actually a 4.0L. Easy way to do that? See if injectors 5 and 6 fire using a lead coming out of the pcm connector and into a NOID light. No signal, so I guess someone bought a 4.0L and reprogrammed it to be 2.5L.

Also, made myself a handy little multiple 4-6 injector led NOID checker to make sure the 4 injectors were firing in the right order. Today I learned what batch injectors are, lol, and that the YJ engines are batch (all 4/6 injectors fire simultaneously).

So, you probably could use a 4.0L pcm in a 2.5l engine anyways.

I'll be installing the new engine sensors soon and replacing vacuum lines to see if it fixes.
 

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Discussion Starter · #18 ·
For anyone who comes across this thread and still has issues, I'm going to tell a tale of the continued efforts to get this Jeep running good:

The vacuum line fix didnt do the job completely.
- Replaced the 02 sensor since it was shot, idle got a tad smoother.
- Checked fuel pressure at the rail and noticed it was low, so I went ahead and replace the fuel pressure regulator since it was cheap.
- Also bought a new fuel filter, because why not? it was cheap too. Then the fun started...

The idiot owners before me put a tiny little mower fuel filter on it (like half the diameter of the right one). Also, to fit on the tiny hose barb of the tiny filter, they cut some evap hose from the gas tank (some small hose rated for very low pressure). That hose was swelling and bulging, about to rupture. Replaced the fuel filter and rubber hose, and now I started getting high fuel pressure, like 50-60 psi.

Once again, I learn not to trust anything the previous own said they did to it. Dropped the tank, took out the sending unit to inspect it and clean the tank. Also blew out the fuel and return lines in case they were dirty/clogged. Im outside checking things in prep to put the tank back in, and I look to double check ill be installing the fuel and return lines correctly how I marked them, and then I notice it.... The fuel line had previously been connected to the return line on the tank sending unit and vice versa!!!

I'll post back after I get the tank back in, but pretty sure having fuel and returns lines swapped plus a tiny fuel filter was a big cause of engine problems.

Also worth noting unrelated to idling rough, I randomly had electrical issues (Power when turning the Jeep on, but total electrical loss when trying to start). Measured a 3v drop going from battery+ to engine fuse box, which ended up being from rusted battery terminals that I cut out and replaced. Fixed electrical issues.

Never ever ever trust previous owners, especially if they say they touched anything on the vehicle!
 

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Discussion Starter · #20 ·
I don't think it can even run that way \£/
I double checked it later to make sure I wasn't crazy by tracing the lines and even squirting some fuel through to see where they came out. The fuel pump ran through the return line to the fuel rail, backwards through the fuel pressure regulator, returning down the main fuel line, backwards through the fuel filter, and back into the tank.

It idled steady, but would practically stall just putting it into drive and trying to move.
 
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