86 2.5 Periodically Runs Poor

[87Warrior]

I am having an issue with my 86 2.5 TBI. It seems as if the temperature drops below 40 degrees the truck will start and idle fine, but once I get about a mile down the road it looses power and starts to buck and miss. This usually goes away after a few minutes, but it lasted for 15 miles a few weeks ago when the temperature was in the teens. When this happens on a cold truck, there is no exhaust steam coming out of the tail pipe.

Short of a cracked exhaust manifold, the truck runs great with plenty of power (for a 2.5 with 175k miles on it). Last year I replaced the O2 sensor to fix the same type of issue with no success. I have also replaced thr CPS as it failed an ohm check and replaced the dizzy cap/rotor and plug wires.

The coolant temperature reads about 140 consistently. I replaced the radiator and tstat (195) this past summer and checked the temp with an IR gun at the tstat housing and got a reading of 198. The heater blows scalding hot air on my feet.

Any suggestions?

Thanks!

[Oyaji]

The cracked exhaust manifold might be allowing pulses of air (and thus oxygen) back into the exhaust system, thus throwing off the oxygen sensor reading.

[Tactical Bacon]

The cracked exhaust manifold might be allowing pulses of air (and thus oxygen) back into the exhaust system, thus throwing off the oxygen sensor reading.

I concur. On start up, the ecu will ignore inputs from the O2 sensors and run off preset tables until the O2 warms up enough to provide accurate input. Then it goes into closed loop and measures fuel trims and air temperatures and whatnot. What's probably happening is the cold weather has caused the crack to increase in size and allow too much air in (too much air and the engine starts dumping fuel since it detects a lean condition) since the metal contracts. Over time, the hot exhaust gases cause the manifold to expand, sealing the crack up to a manageable level.

[cruiser54]

Why is the coolant temp so low? Should be about 190*.

 

Renix O2 sensors go into closed loop very quickly, generally in less than a minute regardless of thermal conditions.

 

Ever taken that braided strap off the firewall and cleaned it up real well as well as scrape the paint off the firewall?

 

I have a hard time subscribing to cold air getting into an exhaust manifold which is under pressure......

[Oyaji]

Why is the coolant temp so low? Should be about 190*.

 

Renix O2 sensors go into closed loop very quickly, generally in less than a minute regardless of thermal conditions.

 

Ever taken that braided strap off the firewall and cleaned it up real well as well as scrape the paint off the firewall?

 

I have a hard time subscribing to cold air getting into an exhaust manifold which is under pressure......

 

 

Good advice as always.

 

But the exhaust system (particularly at the manifold) is not always under positive pressure. It does fluctuate with every opening and closing of exhaust valves, and the rarefaction pulse that follows every compression pulse will indeed create waves of negative (lower than atmospheric) pressure.

 

Remember the reed valves that fed fresh air into the exhaust on old air-injected emissions reduction systems? The above was exactly the principle that made them work. Similarly in tuned exhaust headers, that pressure wave is amplified through the use of harmonics (like a pipe organ) to scavenge exhaust via negative pressure pulses, too.

[cruiser54]

I'm not buying it because that theory is for a one cylinder two stroke engine. On an inline 4, there will always be pressure there. Ever hold your hand over a tailpipe? Did you feel anything pull your hand closer to the pipe? No. 

 

I've done the dollar bill test numerous times to check for burned exhaust valves. If none are burned, the dollar bill will NEVER get sucked up to the pipe. 

[Tactical Bacon]

It may not be sucking the bill towards the manifold but outside air will still move into the manifold between pulses. Why else will an exhaust leak before or near an O2 sensor cause a false lean condition to be read?

[Oyaji]

I'm not buying it because that theory is for a one cylinder two stroke engine. On an inline 4, there will always be pressure there. Ever hold your hand over a tailpipe? Did you feel anything pull your hand closer to the pipe? No. 

 

I've done the dollar bill test numerous times to check for burned exhaust valves. If none are burned, the dollar bill will NEVER get sucked up to the pipe. 

 

No, it is true for all 4-stoke engines no matter how many cylinders. By the time the flow reaches the tailpipe, friction all along the length of the exhaust pipe as well as the baffles and chambers in the muffler will have smoothed out the flow and thus canceled the pulses. (After all, the compression and rarefaction pulses are what defines sound, and one of the main purposes of the exhaust system is to muffle sound.) But the closer you get to the exhaust valve, the more pronounced the pulses are.

 

One of the things I'll do when I get around to fabricating my exhaust is to use an old air-injection reed valve with a venturi in the exhaust to create negative pressure for my substitute PCV, to replace the standard stock setup to the intake manifold. It'll make the exhaust smoke a bit (maybe backfire too?) and will not by any means be road-legal by emissions standards, but it will keep my intake manifold and valves from getting all gunked up with blow-by and oil.

[cruiser54]

It may not be sucking the bill towards the manifold but outside air will still move into the manifold between pulses. Why else will an exhaust leak before or near an O2 sensor cause a false lean condition to be read?

I'm not sure it does. With 3 to 5 more cylinders in the equation, I don't see how a vacuum can be created by 1. 

[Comanchemodder]

.

[cruiser54]

I think we need to stick to the original poster's issue at hand and help him get his Jeep fixed. Practical solutions are what this community is looking for, not heady pontificating responses that just muddy the waters and inflate people's own egos. Isn't that what we're about here? 

[JeepcoMJ]

Well put

[87Warrior]

Tonight I tried something new. After the truck sat for about 9 hours, I started it and immediately put the pedal to the floor several times. Each time it went past 1/2 throttle it would sputter. After a few minutes it would rev up as you would expect.

Why is the coolant temp so low? Should be about 190*.

 

Renix O2 sensors go into closed loop very quickly, generally in less than a minute regardless of thermal conditions.

 

Ever taken that braided strap off the firewall and cleaned it up real well as well as scrape the paint off the firewall?

 

I have a hard time subscribing to cold air getting into an exhaust manifold which is under pressure......

I suspect the coolant sending unit must be bad. The tstat housing read 198 on a hot, running, motor with an IR heat gun, so I know it is running at the proper operating temperature. As I understand it, the Renix 2.5 has a gauge sending unit and a ECU sending unit. I plan to replace them for good measure, but am doubtful they will cure the issue.

I have not removed and cleaned the braided strap from the firewall-head bolt or cleaned up the two dispstick grounds. I plan to do that the first chance I have.

 

I do realize that a cracked manifold can lead to inaccurate O2 readings resulting in poor fuel mileage, etc. I doubt that a 20 degree drop in air temperature would cause so much of an issue with something that regularly has 400+ degree exhaust flowing through it.

[cruiser54]

 

Tonight I tried something new. After the truck sat for about 9 hours, I started it and immediately put the pedal to the floor several times. Each time it went past 1/2 throttle it would sputter. After a few minutes it would rev up as you would expect.

Why is the coolant temp so low? Should be about 190*.

 

Renix O2 sensors go into closed loop very quickly, generally in less than a minute regardless of thermal conditions.

 

Ever taken that braided strap off the firewall and cleaned it up real well as well as scrape the paint off the firewall?

 

I have a hard time subscribing to cold air getting into an exhaust manifold which is under pressure......

I suspect the coolant sending unit must be bad. The tstat housing read 198 on a hot, running, motor with an IR heat gun, so I know it is running at the proper operating temperature. As I understand it, the Renix 2.5 has a gauge sending unit and a ECU sending unit. I plan to replace them for good measure, but am doubtful they will cure the issue.

I have not removed and cleaned the braided strap from the firewall-head bolt or cleaned up the two dispstick grounds. I plan to do that the first chance I have.

 

I do realize that a cracked manifold can lead to inaccurate O2 readings resulting in poor fuel mileage, etc. I doubt that a 20 degree drop in air temperature would cause so much of an issue with something that regularly has 400+ degree exhaust flowing through it.

Progress. Keep in mind that the Coolant temp Sensor grounds at the dipstick tube stud as do other critical sensors. Don't wait to refresh the dipstick tube stud grounds or the braided strap. Actually wouldn't hurt to clean the strap and it's contact point at the engine side also.

 

The theory of exhaust pulses should be at the bottom of your list or better yet, not even on it. 

[nitroxsteve]

I have heard of a crack in the exhaust causing the o2 sensor to read incorrectly. I believe it has to do with the location of the crack. I agree that if the crack is after the point all of the different tubes from each cylinder merge together you will always have some type of pressure in the pipe. But if the crack is in the tube between the cylinder and the collector when that cylinders valves close you will have a dead space with no flow. The scavenging effect of the other cylinders at the collector will cause a suction and draw in fresh air in through the crack. This fresh air is going to affect the o2 sensor. I am not sure if this is the op's problem or not, if he has had the exhaust leak before this problem started its probably not it.

[cruiser54]

He's had the crack for awhile. If the sensor was in a single tube or before all the tubes come together, maybe. 

 

Even so, the OP's issue is new. 

[nitroxsteve]

He's had the crack for awhile. If the sensor was in a single tube or before all the tubes come together, maybe. 

 

Even so, the OP's issue is new. 

agreed

[87Warrior]

I have only had this truck on the road for 2 years, now entering my second winter with it. It had sat since 2004 somewhere in Iowa prior to my saving it. The manifold has been cracked since I got it running. I did have a similar issue last winter that I never did resolve, granted I didn't try that hard. 

[cruiser54]

But it runs okay in warm weather?

 

That kinda rules out all this O2 sensor stuff and exhaust cracks then, doesn't it? 

 

The 4 cylinder TBI engines have an issue with cracking/split vacuum lines near the throttle body. That will screw with the MAP sensor which can cause a drastic change in air/fuel ratio much greater than a failed  O2 sensor.

 

Regardless, do the ground refreshing. You have my instructions? They're for the 4.0 for the most part, but they do share some commonalities and it's worth doing. 

[JeepcoMJ]

Remember a couple of things;

 

An Renix era 4.0 will run in "open loop" mode (a.k.a. factory computer defaults for sensors such as o2 sensors) with a failed o2 sensor.

 

exhaust manifold cracks can help cause the o2 sensors to burn out, and the result can sometimes be that it runs *better* than before with a cracked manifold.

 

It's possible that that is not-so for the coolant temp sensor. I do not recall if there are two or just one on the renix 4 cyl's. I've googled the pics and looked at several engine bays from 80's 2.5's, and it looks like they don't.

 

The Renix 4.0's *do* in some instances (auxillary fan), but the only one that has a primary use directed to the ECM and gauge cluster, IIRC, is the rearward one on the head.

 

The rearward sensor on the head can be one of two; one for dummy/idiot gauges, and one for full gauges.

 

It's possible that there is a safety function the ECM reads off of that to keep it running right, but not likely. Because there are two different sensors with two different gauges available, I do not think that that sensor failing or not can affect how the engine runs.

[JeepcoMJ]

Curiosity for you.

 

 

Two things:

 

#1

Check to make sure CPS wire isn't laying on exhaust manifold. This is the crankshaft position sensor, located in roughly the 11 o'clock position (from driver's seat) of the transmission bellhousing. It runs up and over the back of the exhaust manifold, but is supposed to have a clip that keeps it from laying on it. A CPS getting melted/heated will have higher resistance in the wires, and effect how it runs.

 

#2

Obviousness of checking spark plugs and wires, making sure that they've been done sometime recently (not 10 years ago lol).

With that, remove the distributor cap and make sure that the contacts look nice. Make sure, while you're at it, that the distributor shaft bushings aren't worn out and that it has no side to side play.

[cruiser54]

The CTS on a Renix 4.0 is in the lower left block and the temp sender for the gauge is at the back of the head on the same side.

 

The Renix Jeeps will run acceptably with a bad O2 sensor. many folks wuldn't even notice. A MAP sensor on the other hand has a major part in fuel/air ratio. Second in line is the CTS, followed by the Intake Air Temp sensor, 

 

The reason I'm encouraging the ground refreshing is that the CTS, TPS, IAT, and MAP share the same ground circuit terminating at the engine dipstick tube stud. If the grounds are bad, so are the signals sent to the ECU.

 

We took it upon ourselves to clean the firewall ground, especially on the 4 cylinders at first opportunity, due to the excellent results we got in drivability. 

[JeepcoMJ]

On board with that.

 

Been awhile since I've touched a renix jeep, except to remove the motor and stuff one of my H.O strokers and OBD2 harness in.

[87Warrior]

But it runs okay in warm weather?

 

That kinda rules out all this O2 sensor stuff and exhaust cracks then, doesn't it? 

 

The 4 cylinder TBI engines have an issue with cracking/split vacuum lines near the throttle body. That will screw with the MAP sensor which can cause a drastic change in air/fuel ratio much greater than a failed  O2 sensor.

 

Regardless, do the ground refreshing. You have my instructions? They're for the 4.0 for the most part, but they do share some commonalities and it's worth doing. 

Truck runs great when it is warm.

I have replaced the solid rubber vacuum line block at the TB with individual pieces of vacuum line. This cured a crazy high idle the truck initially had.

I do have the ground refreshing instructions.

 

Curiosity for you.

Two things:

 

#1

Check to make sure CPS wire isn't laying on exhaust manifold. This is the crankshaft position sensor, located in roughly the 11 o'clock position (from driver's seat) of the transmission bellhousing. It runs up and over the back of the exhaust manifold, but is supposed to have a clip that keeps it from laying on it. A CPS getting melted/heated will have higher resistance in the wires, and effect how it runs.

 

#2

Obviousness of checking spark plugs and wires, making sure that they've been done sometime recently (not 10 years ago lol).

With that, remove the distributor cap and make sure that the contacts look nice. Make sure, while you're at it, that the distributor shaft bushings aren't worn out and that it has no side to side play.

CPS wires were clear when I ohm'ed the old CPS and are tied away from the manifold with the new CPS. The contacts in the dizzy cap and rotor were blackened and corroded. I replaced the cap/rotor/plug wires when I replaced the CPS. Distributor shaft was still tight. Plugs are Champion and were new 2 years ago.

 

 

The CTS on a Renix 4.0 is in the lower left block and the temp sender for the gauge is at the back of the head on the same side.

I believe the CTS on the Renix 2.5 is on the intake manifold.

[cruiser54]

Maybe it is on the intake. I've got a brand new one up in the attic with a throttle body and stuff on it. I should go look.

 

Do the sensor ground test while you're at it.

 

 

 

Cruiser’s Renix Sensor Ground Test

This sensor ground circuit affects the CTS, TPS, IAT, MAP, ECU and diagnostic connector grounds. It’s very important and not something to overlook in diagnosing your Renix Jeep as it is common for the harnesses to have poor crimps causing poor grounds. If any or all of the sensors do not have a good ground, the signal the ECU receives from these sensors is inaccurate.

Set your meter to measure Ohms. Be sure the key is in the OFF position.  Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS . The letters are embossed on the connector itself.
Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it runs parallel to the valve cover and also near the MAP sensor mounted on the firewall. If you have an 87 or 88 with the C101 connector mounted on the firewall above the brake booster, wiggle it, too.

You want to see as close to 0 ohms of resistance as possible. And when wiggling the harnesses/connectors the resistance value should stay low. If there is a variance in the values when wiggling the wires, you have a poor crimp/connection in the wiring harness or a poor ground at the engine dipstick tube stud. On 87 and 88 models, you could have a poor connection at the C101 connector as well.

Revised 06/12/2012