Bad MPG, Runs rough, Gas smell......and more

Alright, my beast is being a PITA yet again. Currently its running extremely rough. Similar to how an old 454 with a carb would run, chugging i guess would be the correct word. There's a strong gas smell constantly and I'm getting horrid MPG, around 7-10 depending on how i drive. Also I've lost 30% of my power if not more.

I originally though it was an exhaust leak. But from the end of the manifold back is custom made and less than a year old. I looked at the exhaust manifold from the top and bottom and it appears to have a good seal. I have no idea what in the could be wrong. Does anyone have any ideas? Could there be a bad sensor or something that isn't tripping the code light on the dash?

One little side note, right before this problem started there was an issue to where the vacuum line from the MAP sensor to the throttle body would kick out (of the throttle body) if I stepped on the gas to get through these weird power-loss hesitations. They felt like extremely rapid compression brakes and would only last 1-2 seconds. While the line was still plugged in my gas mileage seemed fine, but i was low on power and get the hesitations if I tried going more than 15% throttle. When the line did pop out the gas smell would be stronger and the motor would feel like it had less vacuum pressure. But I would regain a good amount of power and I wouldn't have the hesitations. I replaced the vacuum line going from the MAP sensor to the throttle body with a slightly larger diameter rubber hose due to braking it while looking at it. Now its constantly running rough like it was when the MAP sensor was unplugged.

It's a 87 4.0 renix

sounds like a misfire.

what year is it, miles, engine, etc.....

When is the last time you insalled: new plugs, cap rotor or coil bar.....

Probably nothing to do with the vac or map sensor......

could be a bad injector mine did that too

Its a 87 with appx 250k on the motor. How can you test to see if your injectors are bad?

You can get a long screw driver and put in on each injector individually as it's running and put your ear on the handle. You want to see if it's clicking . Kind of an old school method but it worked for me.

I would pull the spark plugs and examine their condition. That should tell you if the problem is isolated or across all cylinders.

Things that make your XJ run rich: bad o2 sensor (only good for 60k miles); exhaust leak upstream of o2 sensor; bad MAP; bad MAT; bad injector(s); bad fuel pressure regulator; bad o-ring seals; intake manifold leak; bad CTS (in the block, under the manifolds); bad thermostat.

All of your sensors can be tested with a multimeter. BOOKMARK this website: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm it has everything you need to know about Renix sensors and testing them.

That vacuum line to the map sensor needs to be a tight fit. Be sure that it is, even if that means replacing that vacuum line.
The map sensor contributes to determining fuel/air mixture. It can be tested with a meter and the procedure.

You need to isolate the "strong smell of gas". It is likely related to your issues. And it also is telling you that you have a fire hazard under the hood.

How old/how many miles on your fuel injectors?

birchlakeXJ said:

That vacuum line to the map sensor needs to be a tight fit. Be sure that it is, even if that means replacing that vacuum line.
The map sensor contributes to determining fuel/air mixture. It can be tested with a meter and the procedure.

You need to isolate the "strong smell of gas". It is likely related to your issues. And it also is telling you that you have a fire hazard under the hood.

How old/how many miles on your fuel injectors?

Click to expand...

Damn good point--read JP Magazine's article on fireproofing the 4.0: http://www.jpmagazine.com/techarticles/engine/154_0811_preventing_jeep_engine_fires/index.html

Everything you described sounds like when my MAP sensor went south. I would replace the vacuum line with a similar size line to see if it works, then check the voltage reading from it.

Just replaced the MAP sensor last night, all the problems are still present. I noticed something kinda weird. The grommet that the vacuum line from the map sensor connects to in the throttle body has 2 holes. One has the vacuum line plugged in and the other is just an open hole. I plugged it hoping that the problem was just a vacuum leak, it didn't work. So whats that 2nd hole to? And does it matter if I have the map sensor plugged into the top or bottom hole in the throttle body?

The top is plugged, the MAP vacuum line draws from the bottom port.

Alright, plugged it last night. After joe replied I realized I had the plug and MAP sensor in the wrong holes. Switched them and the gas smell has gone and I have power back. I'm still having issues, inconsistent rpm with occasional jumps. When I start to accelerate sometime it will bog down and then just jump and start moving. Also it bogs down pretty bad when trying to accelerate slowly in low rpm while in 3rd and 4th.

Damn Renix--I was all set to tell you to drain the PCM's memory when I remembered to check your year again. LOL!

Ok, I'm thinking you have lots of black fluffy carbon on your plugs and such.

Take here for a run at full operating temp for about 30 minutes and see if she clears her throat.

If that doesn't help, locate some BG44K and run it through the tank.

Renix does store recent sensor data!!!! I can confirm that from hands on experience and from the OEM manuals!!!!! So if it has old bad data, like a funky TPS that was just replaced, or calibration data, it does do wonders to clear the memory!!! The Renix TCU also stores old operating parameter data as well for the transmission!!!! I have cleared Renix TPS idle memory with 3-4, 10 second on, 10 second off cycles, with our starting it!!!!!

joe_peters said:

Damn Renix--I was all set to tell you to drain the PCM's memory when I remembered to check your year again. LOL!

Ok, I'm thinking you have lots of black fluffy carbon on your plugs and such.

Take here for a run at full operating temp for about 30 minutes and see if she clears her throat.

If that doesn't help, locate some BG44K and run it through the tank.

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Being an 87, add the EGR valve, and EGR solenoid valve , et, al, to the check list. Sounds like a noisy TPS signal? If it is Auto, AW4, test both sides of the TPS for bad spots from idle to WOT! Either side can be bad by itself and cause issues. Give us an update, on all the remaining issues now that the MAP vac line is fixed!!!

250K! Congratulations. First thing to do is check your compression. Should at least be over 100PSI. If compression isn't good enough, you are wasting your time (and maybe money) on your beast.

winterbeater said:

250K! Congratulations. First thing to do is check your compression. Should at least be over 100PSI. If compression isn't good enough, you are wasting your time (and maybe money) on your beast.

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I found a guy down the street that has over 350,000 miles on his.

Ecomike said:

Renix does store recent sensor data!!!! I can confirm that from hands on experience and from the OEM manuals!!!!! So if it has old bad data, like a funky TPS that was just replaced, or calibration data, it does do wonders to clear the memory!!! The Renix TCU also stores old operating parameter data as well for the transmission!!!! I have cleared Renix TPS idle memory with 3-4, 10 second on, 10 second off cycles, with our starting it!!!!!

Click to expand...

We will have to agree to disagree on this issue.

Yes, the Renix ECU has some LIMITED memory for fault storage, that can only be read by a DRB scanner--KAM, Keep Alive Memory--and it will track the number of times a fault was found, but that doesn't effect the ECU's actions like stored sensor data does in the PCMs. Wiping KAM in the Renix just resets the counters.

joe_peters said:

We will have to agree to disagree on this issue.

Yes, the Renix ECU has some LIMITED memory for fault storage, that can only be read by a DRB scanner--KAM, Keep Alive Memory--and it will track the number of times a fault was found, but that doesn't effect the ECU's actions like stored sensor data does in the PCMs. Wiping KAM in the Renix just resets the counters.

Click to expand...

I just can't agree to that!:laugh:

I see your point, that KAM is not what I once thought it was, but I have seen repeated impirical evidence on my Renix jeeps that it can and does store funky data, that is cleared by a battery disconnect, or 3-4 on-off, no start cycles of the ignition switch, that clears old memory of bad TPS idle settings. What all it is storing, I am not sure, I suspect it is at least a prior min value for idle on the TPS from prior cycles, but I can prove it (yet). It would also make sense from an engine management perspective that it would store a few prior min idle TPS voltages. What I do know is that clearing the memory after resetting a TPS idle voltage, calibration step, works better than not clearing memory, especially after the ECU has seen really bad idle voltage settings just before the new TPS was installed.

Either that or it confuses the morphidite electrons and gremlins in the wiring, and they move on to find a new home! LOL.

So, it is a like an electron evictions notice? I like that!