XJ Running Rich

1988 4.0 AX15 50k miles.

Noticed a pretty decent build up of black soot on my exhaust dump, similar to that on my diesel. Don't think it's oil, doesn't burn any. What usually causes this and besides bad gas mileage what else can it hurt? 02 sensor? It's been running really good.

Welcome to the "running rich" club.

The most expensive things a rich mix hurts are the O2 sensors (soots them up), the catalytic converter (overheats it and burns it out), and of course the gas mileage (empties your wallet every time you gas up).

I was just looking at the manual and see (at least on my 2001) the MAP is connected to the intake manifold by a rubber right-angle nipple. Several others have already sprung leaks and been replaced, so tomorrow when it's light out I'll check this one to see if it is split and leaking also. I never noticed it because it is under the MAP and out of sight.

HTH

Spearfisher XJ said:

1988 4.0 AX15 50k miles.

Noticed a pretty decent build up of black soot on my exhaust dump, similar to that on my diesel. Don't think it's oil, doesn't burn any. What usually causes this and besides bad gas mileage what else can it hurt? 02 sensor? It's been running really good.

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It may be nothing but normal. Running good as in steady idle etc?

Make sure your intake manifold bolts are tight and spray throttle body cleaner along the intake and vacuum hoses at idle, paying particular attention the the hose from the throttle body to MAP.

O2 sensors only last about 100K miles at best. You can test it.

Last tune-up with plugs, wires, cap and rotor?

Ever done a ground refreshing on that old Renix?

Your computer while still in open loop looks at the coolant temp sensor first to determine startup mixture. If you have a bad coolant temp sensor it can be thinking it is excessively cold all the time and just dumping in the fuel.

Pull the spark plugs and see if they have black dry soot on them too. If yes it is running rich, and can burn out the Cat converter.

You can test the O2 sensor live with an old style analog volt meter to see if it is indeed running rich, or if the O2 sensor is even working.

What are MPGs so far? And driving conditions?

joejeep92 said:

Your computer while still in open loop looks at the coolant temp sensor first to determine startup mixture. If you have a bad coolant temp sensor it can be thinking it is excessively cold all the time and just dumping in the fuel.

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That is more common with the newer Jeeps, especially ODB-II. Renix seems to be able to go closed loop in a deep freeze if the O2 sensor and its internal electric heater are working.

Make sure your engine is warming up to operating temperature. If the ECU thinks the engine is cold it will keep the mixture rich--like driving around with the choke on.

joe_peters said:

Make sure your engine is warming up to operating temperature. If the ECU thinks the engine is cold it will keep the mixture rich--like driving around with the choke on.

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You may have missed a thread by old_man early last year were he confirmed that a stuck open T-stat on Renix made his stroker eat gas, even in closed loop! It was an interesting thread, and a bit of a surprise how much difference it made.

Ecomike said:

You may have missed a thread by old_man early last year were he confirmed that a stuck open T-stat on Renix made his stroker eat gas, even in closed loop! It was an interesting thread, and a bit of a surprise how much difference it made.

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Yep, there is a reason for "normal operating temperature".

Another common source on the older renix engines is a bad air charge sensor. You can test it with a simple ohmmeter. Most people don't even know where it is located.

old_man said:

Another common source on the older renix engines is a bad air charge sensor. You can test it with a simple ohmmeter. Most people don't even know where it is located.

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I've had good results removing the IAT sensor and cleaning the contacts with a toothbrush and throttle body cleaner. I did a side by side test and found a crudded up one varied in ohms reading from a clean one by 10%.

For newbies down the road, Air Charge sensor (ACT-?), is also called Intake Air Temperature sensor, or Manifold Air Temperature sensor, MAT and IAT are commonly used. The aftermarket computers use different names, so it is handy to know all the name versions.

Ecomike said:

For newbies down the road, Air Charge sensor (ACT-?), is also called Intake Air Temperature sensor, or Manifold Air Temperature sensor, MAT and IAT are commonly used. The aftermarket computers use different names, so it is handy to know all the name versions.

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I've got a BRAND NEW one Mike.

cruiser54 said:

I've got a BRAND NEW one Mike.

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Congrats! Sounds like you are getting a charge out of it already, LOL.

Mike (hypoid) and i discovered that a bad coolant temp sensor on his buddies renix was causing a rich condition by not letting her go into closed loop... if you can get your hands on a scan tool you'll be bux ahead

cruiser54 said:

I've got a BRAND NEW one Mike.

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Hold onto it, they are OBSOLETE now.

So why are all the retail parts stores and ebayers showing these for 88-96?

http://www.ebay.com/itm/New-Omix-IA...pt=Motors_Car_Truck_Parts_Accessories&vxp=mtr

This is just an example.

Ecomike said:

So why are all the retail parts stores and ebayers showing these for 88-96?

http://www.ebay.com/itm/New-Omix-IA...pt=Motors_Car_Truck_Parts_Accessories&vxp=mtr

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COOL--two years ago NOBODY had them, and the only ones 5-90 found that had the same resistance/temp values was a 3/8" NPT piece from Marren Fuel Injection. That is the one I'm running now in a bung welded into my aftermarket intake tube.

Quick search of NAPA and O'Reilly's still shows no MAT/IAT--it would appear Omix has filled a void.

joe_peters said:

COOL--two years ago NOBODY had them, and the only ones 5-90 found that had the same resistance/temp values was a 3/8" NPT piece from Marren Fuel Injection. That is the one I'm running now in a bung welded into my aftermarket intake tube.

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But you checked last week and you implied in your post that the 91 curve is completely different from the 87-90? You never gave the details, just said you looked them up and "they were totally different"?

So I am still wondering what is different, what did you see that was different? Was it the temp-resistance table or the connectors or what?

Ecomike said:

But you checked last week and you implied in your post that the 91 curve is completely different from the 87-90? You never gave the details, just said you looked them up and "they were totally different"?

So I am still wondering what is different, what did you see that was different? Was it the temp-resistance table or the connectors or what?

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Yep, we were posting about that guy's 88, and you asked if the 91 MAT could be used with a different connector, so I checked the 93 manual as the 91-93 and 94-96 all use the same MAT, and the OHMs did not match up at all.

Now, quick and dirty here is a range of resistance values from the RENIX manual:

@40 deg. F 7,500 ohms
@70 deg. F 3,400 ohms
@100 deg. F 1,600 ohms.

If you look at the specs on NAPAprolink for the 91-93 and 94-96 MAT, ECH TS5014:

@77 deg. F 10,000 ohms.

If you want I will dig out the 93 specs from the manual, but it is already clear that the resistance values are not the same, and unless Omix has magic fairy dust to use with their MAT I don't think it is going to fit 88 through 96 as their ad suggests.

These are the specs from the 95 manual--same sensor:

@ 32 deg. F 29,330 to 35,990 ohms
@ 50 deg. F 17,900 to 21,810 ohms
@ 77 deg. F 9,120 to 10,880 ohms (matches nicely with NAPA's rated 10,000 @ 77 deg. F)
@ 104 deg. F 4,900 to 5,750 ohms

Now, as I said last week, there is a BIG difference in the sensors and now I will say OMIX's ad is BOGUS. :wave1:

Actually, that is an eBay ad so Omix may not be the villian--I sent the eBay vendor a message that the ad is bogus.