98 GC Limited 4.0 : runs OK cold then bogs/loss of power when hot - JeepForum.com
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post #1 of 18 Old 05-31-2018, 03:29 PM Thread Starter
FlyingFatCat
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98 GC Limited 4.0 : runs OK cold then bogs/loss of power when hot

Folks,

My 98 GC limited 4.0 has about 150,000 on it. What I have now is the jeep runs great when it's cold. At morning startup and cold it has the usual immediate throttle response and good acceleration. When I get down a block or so and stop, then the throttle response goes flat and boggy. Especially when the transmission shifts into another gear, all I can say is the power just 'mushes' out and is no longer responsive, think of like pulling a large trailer load.

I'm out of town away from my tools and spares. Any suggestions on what to first check? TPS is fairly new. Again, this only happens once it's warmed up then is lack of power and mush time.

Also, no codes or CEL.

Thanks in advance,

Chip
---


- 1998 Grand Cherokee Limited 4.0L ZJ : Like new (07/14)
- 1981 CJ-7: 304 AMC, tons O' mods, 300,000+ miles (mid 80's)

Can you re-build a T-176 tranny in the kitchen sink? = yep.
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post #2 of 18 Old 05-31-2018, 04:54 PM
jtec
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first I would SCAN for codes, key on off is lacking and not all codes turn on CEL.
Scans are free at manypart stores...

I would start with BASICS...
plugs remove and 'read' them Wet,Dry, Black, Tan, damaged.
Fuel pressure with a gauge.
Check for vacuum leaks - google finding vac leaks with carb cleaner...

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 18 Old 05-31-2018, 07:19 PM
GOLDWING
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^^^^ What jtec said.

Also put a vacuum gauge on it at cold start and compare with the reading when it gets warm and bogging. A noticeable difference may point to a clogged cat.

GW

"The Dude Abides"
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post #4 of 18 Old 06-01-2018, 05:16 AM Thread Starter
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Thanks. I understand the basics and will be doing all of that hopefully today. I thought there may be something different due to the fact it runs 'fine' cold and boggs when warmed up. I was just overthinking the basics due to the fact running cold fine issue. If plugs were bad why would that cause it to run bad when hot?

Cat or vac leak when warmed up makes sense. Someone said pull the upstream OX sensor also.

Been working on cars and engines for 30 years, this is my first fuel injected and multiple sensor vehicle. I'll learn this also.

Thanks, will let you know what I find out.

- 1998 Grand Cherokee Limited 4.0L ZJ : Like new (07/14)
- 1981 CJ-7: 304 AMC, tons O' mods, 300,000+ miles (mid 80's)

Can you re-build a T-176 tranny in the kitchen sink? = yep.
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post #5 of 18 Old 06-01-2018, 08:49 AM
jtec
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It could be
running RICH and when PCM takes over it tries to adjust and cannot achieve correction.
When temp reached and systems ie EVAP begin to operate system goes too lean.
A sensor (input) is an issue and when closed loop it affects PCM.

SCAN for codes even if al you can do is key on key off its a start.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #6 of 18 Old 06-01-2018, 11:42 AM Thread Starter
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OK, with the limited time i have until later today here is my stragegy:

Completed:
- MAN pressure using OBD II reader = 13" at idle = need specification
- pulled connector to the Intake Air Temp sensor = no change. reset code
- OX sensor voltage readings as shown in variable 'OS1' = fluctuating between ~0.1 and 0.8 volts at idle. I expect this is the voltage input to the PCM?

To Do:
- Pull throttle body and do a complete cleaning of all parts, IAC, etc.
- Check plugs, wires, cap and rotor again. Replaced not too long ago with proper Champion copper plugs, brass distributor cap, NAPA wires
- Check for intake manifold/hose leaks.
- Some how have the catalytic converter tested, that's a big one for me
- do a test on the MAP sensor using vacuum pump
- replace OX sensor(s)

Thanks for all you folks input so far. Again, I'm out of town away from my tools and vac gauge and other diagnostic goodies, so I limited to what I can do here. Bought a set of metric sockets from Harbor Freight cheap, now I have an on-board source of some tools.

A couple of questions:
I know that my OAT sensor inside the front grill is good but reads wrong temp on the overhead display. It does change though. I assume the OAT is not tied into the PCM if I remember right using the FSM, which I do not have with me either.

Due to tight budget can I replace just the upstream OX sensor or do I have to change both?

I wish I was home, sure wold be a lot easier.

Thanks again all, cheers,
---

- 1998 Grand Cherokee Limited 4.0L ZJ : Like new (07/14)
- 1981 CJ-7: 304 AMC, tons O' mods, 300,000+ miles (mid 80's)

Can you re-build a T-176 tranny in the kitchen sink? = yep.
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post #7 of 18 Old 06-01-2018, 11:48 AM Thread Starter
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Quote:
Originally Posted by jtec View Post
It could be
running RICH and when PCM takes over it tries to adjust and cannot achieve correction.
When temp reached and systems ie EVAP begin to operate system goes too lean.
A sensor (input) is an issue and when closed loop it affects PCM.

SCAN for codes even if al you can do is key on key off its a start.
Interesting. Not quite sure how the whole thing works being a Holley only man for many years. Is there a link somewhere so I can read up on that? If I had the money to throw at it I would replace them all. I assume when cold it's in open loop and warmed up in full closed loop? I'm also an engineer and OCD so I learn pretty quick!

BTW: My OBII scanner shows NO codes.

Cheers
---

- 1998 Grand Cherokee Limited 4.0L ZJ : Like new (07/14)
- 1981 CJ-7: 304 AMC, tons O' mods, 300,000+ miles (mid 80's)

Can you re-build a T-176 tranny in the kitchen sink? = yep.
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post #8 of 18 Old 06-01-2018, 12:12 PM
jtec
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OK it seams you can see live data
you want to see if in closed or open loop.
ECT coolant temp ?
o2 sensors are going from lean to rich- check open or closed loop...
Fuel trims long and short...

FUEL PRESSURE - might be able to utilize Fuel trims for a idea...
Instead of just pulling o2 sensor - rent/borrow a backpressure gauge at part stores.


OAT = AAT Ambient Air Temp

Erasing codes also erases stored data....emission monitors, freeze frame

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #9 of 18 Old 06-01-2018, 01:37 PM
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Fuel system operation and open/ closed loop operation all are explained well enough in the FSM (factory service manual).

If it runs fine when cold and bad when up to warm enough, it definitely means there is a sensor/ sensors that make it run bad when the system is in closed loop, monitoring all sensor inputs.




Open loop modes are engine warm up, acceleration and deceleration, and wide open throttle. Open loop modes skip O2- sensor input, and some other depending on open loop mode. Engine operates according to preprogrammed fixed parameters when in open loop.
Idle and cruise are closed loop, when all sensor inputs are monitored and PCM adjusts parameters accordingly. Closed loop modes are adaptive, so that the PCM can adjust multiple parameters according to sensor inputs.




Very often if the engine runs fine when cold (idle, cruising, acceleration/ deceleration), or when accelerating hard enough with a warm engine, the problem is with the upstream O2 sensor(s).
Other issues can be faulty coolant temp sensor, throttle position sensor, manifold absolute pressure and intake air temp.
Do not waste money throwing in new sensors to replace ones that may be operating just fine - diagnose! Do what has been suggested regarding all possible faults.



All this being said, I've had it happen TWICE on two completely different vehicles (first was an Opel that used mid-90s Bosch Motronic SMPI fuel injection system) and then a couple years ago on my 5.9 ZJ - where upstream O2 sensor failed in a way that it gave no check engine light. On my ZJ where I could check the OBD2 data, even the live data feed was showing somewhat reasonable voltages. Both cases solved with swapping in new O2 sensors.



OEM NTK O2 sensors for the ZJ cost next to nothing in Rockauto, so those are the ones I can suggest buying just to be sure.
Also do keep in mind O2- sensor electrical wiring is known failure point where hot exhaust causes shorting inside the wire harness.

1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...


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***Under construction***
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post #10 of 18 Old 06-04-2018, 06:57 AM Thread Starter
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Thanks all very much for the infomation. I looked up the 'engine modes' under the 12 things thread. Makes more sense now. OK here is what I have done and the effects so far, I'm getting there I hope.

- Disconnected IAC connector
- Ran Jeep, very little change
- Removed Throttle Body
- Cleaned Throttle Body completely, I mean like new.
- Cleaned IAC motor at the sealing end. Not just solvent, I also used 000 steel wool and polished the metal tip/sealing part until it was nice and shiny. I don't see that in some videos.
- Cleaned where the IAC plunger/end meets inside the Throttle Body also with steel wool
- Re-assemble

What a difference. When cold it would run a lot better in short trips. It even shifted better. It also now maintains proper idle with or without the A/C engaged. I thought I had it fixed until yesterday when I went for a longer trip. Back to loss of power, mushy especially after transmission shifts gear.

Don't know if the PCM finally re-learned or what. Idle quality is still good though.

I also think I have an exhaust header leak when cold. Her that will throw off the OX senders thinking a lean condition to the PCM.

So next up
- Ordered OX senders from Rock Auto (thanks for the suggestion), Advance wanted ~$57 a piece! RA about ~$24 each for NTK's.
- fuel pressure check
- CAT check
- exhaust manifold check

Sill, it's snappy and runs fine cold so as said some sensor is causing the PCM not to operate correctly when in a closed loop mode. I'll do some more reading and research on that. With that said, I would not think it's plugs or CAT, should run bad when cold also?

Cheers
---

- 1998 Grand Cherokee Limited 4.0L ZJ : Like new (07/14)
- 1981 CJ-7: 304 AMC, tons O' mods, 300,000+ miles (mid 80's)

Can you re-build a T-176 tranny in the kitchen sink? = yep.
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post #11 of 18 Old 06-04-2018, 08:41 AM
jtec
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data cold and data warm

have you checked the ECT?
Look at o2 voltages after COLD soak... is it fixed? @.450
What is TPS at idle?
ANd I think fuel pressure is on 2 do list?
if not fuel trims.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #12 of 18 Old 06-04-2018, 02:13 PM Thread Starter
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Quote:
Originally Posted by jtec View Post
data cold and data warm

have you checked the ECT?
Look at o2 voltages after COLD soak... is it fixed? @.450
What is TPS at idle?
ANd I think fuel pressure is on 2 do list?
if not fuel trims.
Question: Do you want me to look at non-running 'just key on' O2's or running O2's just after start in a cold soak?

- ECT [EDIT] live data does match the dash indication

- O2 readings currently vary between 0.2xx to 0.8xx volts at idle, cold or hot

- TPS = I will check again, forgot to write that one down

-FUEL TRIMS: Do not quite understand fuel trims yet however I can see them in the live data, is there an article somewhere I can read? My thought process is that it would seem that those Fuel Trim values are what influences the PCM and there by the pulse width on the injectors to control fuel delivery in a closed loop operation, correct? The current values are changing in time with the O2 sensor readings. New O2 sensors should be here later today.

This debug is getting interesting. Thanks folks again for all your help.

- 1998 Grand Cherokee Limited 4.0L ZJ : Like new (07/14)
- 1981 CJ-7: 304 AMC, tons O' mods, 300,000+ miles (mid 80's)

Can you re-build a T-176 tranny in the kitchen sink? = yep.

Last edited by FlyingFatCat; 06-04-2018 at 02:57 PM.
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post #13 of 18 Old 06-04-2018, 02:17 PM
jtec
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busy day no time
ECT is eng coolant temp as seen by PCM

https://www.obd-codes.com/faq/fuel-trims.php

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #14 of 18 Old 06-04-2018, 02:43 PM
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Fuel trim explanation.

https://youtu.be/Oa9PAr79hbQ

YW

GW

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post #15 of 18 Old 06-04-2018, 02:44 PM Thread Starter
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First try at posting images here...

- Idle Air Control (IAC) motor. Cleaned with solvent then 000 steel wool. You can see the seating surface of the end is worn. I polished it more than shown here and also inside the mating surface inside the Throttle Body (TB).

Last image is of the Intake Manifold Temp sensor after cleaning. The blue tip item is the thermistor that detects the air temp. It *was* coated with debris.

Cheers
---
Attached Thumbnails
IMG_20180601_165625.jpg   IMG_20180601_160617.jpg  

- 1998 Grand Cherokee Limited 4.0L ZJ : Like new (07/14)
- 1981 CJ-7: 304 AMC, tons O' mods, 300,000+ miles (mid 80's)

Can you re-build a T-176 tranny in the kitchen sink? = yep.
FlyingFatCat is offline  
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