Runs rough when cold

I know, this topic has been beaten to death but I'm out of ideas. Here are my symptoms; Starts up decent and idles ok (slight miss), step on the gas and it acts like a 2 stroke with a clogged jet.... stumbles and then lean pops when full throttle. This goes on until its warm then its fine. Now, what I've done:
1. New plugs (yes, copper)
2. New wires (Bosch), cap/ rotor (was new MSD but I swapped to NAPA premium)
3. New fuel pump (Bosch) and regulator (In-tank.. Thank you 96)
4. Cleaned throttle body, IAC, MAP, and MAT sensors
5. Checked all vacuum lines
6. Replaced both O2 sensors
7. New Yellow fuel injectors
8. New Fuel filter
1996 XJ, 4.0, Auto

Note** I tested the fuel pressure and its at about 39PSI while running. After I turn it off it bleeds to 0 pretty quickly. I pulled the fuel rail and verified that the injectors weren't leaking down.

Any pointers would be appreciated. Thanks!

splatvonzipper said:

I know, this topic has been beaten to death but I'm out of ideas. Here are my symptoms; Starts up decent and idles ok (slight miss), step on the gas and it acts like a 2 stroke with a clogged jet.... stumbles and then lean pops when full throttle. This goes on until its warm then its fine. Now, what I've done:
1. New plugs (yes, copper)
2. New wires (Bosch), cap/ rotor (was new MSD but I swapped to NAPA premium)
3. New fuel pump (Bosch) and regulator (In-tank.. Thank you 96)
4. Cleaned throttle body, IAC, MAP, and MAT sensors
5. Checked all vacuum lines
6. Replaced both O2 sensors
7. New Yellow fuel injectors
8. New Fuel filter
1996 XJ, 4.0, Auto

Note** I tested the fuel pressure and its at about 39PSI while running. After I turn it off it bleeds to 0 pretty quickly. I pulled the fuel rail and verified that the injectors weren't leaking down.

Any pointers would be appreciated. Thanks!

Click to expand...

It's difficult to determine without further testing. Could be a faulty fuel pump or something electrical connected to the fuel pump, electrical connections to fuel injectors or something not directly related to the fuel system such as TPS, temperature sensors, etc.

Electrical connections to the fuel pump require a steady 12 volts, good grounding, etc..

Zero PSI with a cold engine is normal. An engine that has been running and then turned off, there should be fuel pressure in the fuel line that holds a min. psi for a while.

There are leak down tests that can be performed using a T adapter in the fuel line, after the fuel rail. Normally this is how mechanics test to determine which part of the fuel line is not holding pressure immediately the engine is shut off, the engine is allowed to first warm up to operating temps, then the leak down tests are performed.

You might be able to make your own at some spare fuel line and fittings ... fuel line T adapters you purchase for the Jeep look something like.

http://i1240.photobucket.com/albums... Town and Country/Miller-6539_zps7de79b64.jpg

https://w05.dealerconnect.chrysler.com/service/mds2002/serviceInfo/en_US/800128b1.gif

Muddzz said:

It's difficult to determine without further testing. Could be a faulty fuel pump or something electrical connected to the fuel pump, electrical connections to fuel injectors or something not directly related to the fuel system such as TPS, temperature sensors, etc.

Electrical connections to the fuel pump require a steady 12 volts, good grounding, etc..

Zero PSI with a cold engine is normal. An engine that has been running and then turned off, there should be fuel pressure in the fuel line that holds a min. psi for a while.

There are leak down tests that can be performed using a T adapter in the fuel line, after the fuel rail. Normally this is how mechanics test to determine which part of the fuel line is not holding pressure immediately the engine is shut off, the engine is allowed to first warm up to operating temps, then the leak down tests are performed.

You might be able to make your own at some spare fuel line and fittings ... fuel line T adapters you purchase for the Jeep look something like.

http://i1240.photobucket.com/albums... Town and Country/Miller-6539_zps7de79b64.jpg

https://w05.dealerconnect.chrysler.com/service/mds2002/serviceInfo/en_US/800128b1.gif

Click to expand...

Thank you for your time Mudzz. I know for sure that the leakdown is fast after i installed the new pump and regulator. All things point to a faulty pump but Im hesitant to replace the brand new unit i just put in.

Injector test (98 Cherokee)

Disconnect the fuel injector wire harness connector from the injector. Place an ohmmeter across the injector electrical terminals. Resistance reading should be approximately 12 ohms 61.2 ohms at 20°C (68°F).

Your problem sounds as if it is during warm up, the PCM is in open loop mode vs closed loop.

During open loop some sensors are not being monitored or data fed to the PCM.

When the ignition is turned on, the PCM uses ambient temperature, coolant temperature, and barometric pressure sensor inputs to determine the correct air/fuel mixture ratio for starting the engine. In some applications, critical data like barometric pressure or altitude might be stored in an adaptive memory built into the PCM so the engine will immediately start.

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To determine if there is some sort of fuel leak near the fuel rail or somewhere in the fuel line to the gas tank. Install a T adapter and pressure tester then clamp off the fuel line between the fuel line and T adapter. Then perform a leak down test. Zero PSI after the engine is warm and shut off would indicate a leak in the fuel line between the T-adapter and fuel pump / fuel tank.

Leak down tests for a 98 ... Fuel pressure should not fall below 30 psi for five mins.

fuel rail ==== (pinch test 1)=== T adapter ==(pinch test 2)===================== Fuel tank

The T adapter connects to fuel pressure tester

Test One - run engine to operating temp the shut off the engine, pinch the fuel line before the T adapter, gauge should read a min. of 30 psi for at least 5 mins. if zero indicates the problem is somewhere in the fuel line or fuel tank.

Test Two - is the same as the first test, the only difference is the fuel line is pinched after the T adapter .. Fuel pressure should not fall below 30 psi for at least five mins. If zero psi indicates the problem is with the fuel rail ..

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There is also a fuel pump capacity test ... we use to just use a jar with measurement markings, then measure the amount of fuel in the jar after one min. of cranking the engine. (coil wire disconnected) I can't remember off hand the flow rate for the Jeep's fuel pump.

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Start then Stall

Possible causes ...

– engine coolant temperature
– manifold absolute pressure
– intake air temperature
– engine revolutions
– throttle position

Fuel pressure isn't monitored by the PCM so it doesn't know if their is enough pressure or fuel capacity in the fuel line ...

I would have to look at my notes, for more details, the PCM during warm up reads the CTS as well as other sensors to determine a fuel ratio. A faulty CTS during warm up potentially can be the cause starting and stalling problems. If intermittent it could be e.g. a loose wire.

This editor isn't functional after x number of mins. and won't allow editing, corrections of typos and adding additional information.
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It's possible there is more than one problem e.g. with a sensor and a leak in fuel delivery.

As stated previously the PCM looks at data from the CTS and other sensors during warm up and determines a fuel strategy, an a/f ratio (a pulse width for the injectors). If e.g. the CTS when cold isn't working correctly and only works at higher temperatures (closed loop mode, o2 sensors go on-line as primary input to help determine an a/f ratio) , this can potentially can cause stalling problems while the engine is warming up.

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As for zero psi .. I haven't worked on a 96, in 97 there were changes made to the fuel delivery system.

The 98 XJ, has two drain back valves, one in the fuel pump the other is in the filter/regulator. When performing a leak down test a rapid loss of fuel pressure indicates a problem with the drain back valve installed in the fuel regulator/filter vs the drain back valve in the fuel pump.

The purpose of a drain back valve is to hold fuel in the fuel line, so during engine start (hot or cold) you will have instant fuel pressure from the fuel pump to start the engine. Symptoms of a faulty drain back valve is usually a hard start, or needing to prime the fuel line.

One method to diagnose faulty sensors. Pull a sensor connector while the engine is off or while the engine is running e.g. Pull the connector from the CTS before engine start and observe any changes while the engine is running during engine warm up (open loop). No or little change may indicate a faulty sensor during engine warm-up.

If you have a scanner with live readout you can observe temperature readings from the scanner. A faulty sensor may have erratic readings such as voltages, temperatures, etc.

There are other tests for sensors and their connections (plugs, wires, etc.) that can be performed with a voltmeter or DMM while the sensor is installed or removed.

My '99 sport has been doing the same thing, at cold start, it idles ok, when pedal is pushed 1/4 way to get going from a stop it feels like it loads up and stumbles. If you gas it, like 1/2 pedal or more it clears right up and goes. Only when cold though.......

I read that you can clear the PCM memory by un-hooking battery and holding both +,- cables to together for a couple minutes to drain it. Then after the next 20 start ups it re-configures the input values? Is this worth trying at all, or would a CTS (coolant temp sensor, I'm guessing) maybe faulty causing this to happen only at cold start ups?

Check the MAP sensor, check the CTS on the t-stat housing.

Talyn said:

Check the MAP sensor, check the CTS on the t-stat housing.

Click to expand...

Is there specific ohm values I should be looking for to tell if its bad or not?

little squirt said:

My '99 sport has been doing the same thing, at cold start, it idles ok, when pedal is pushed 1/4 way to get going from a stop it feels like it loads up and stumbles. If you gas it, like 1/2 pedal or more it clears right up and goes. Only when cold though.......

I read that you can clear the PCM memory by un-hooking battery and holding both +,- cables to together for a couple minutes to drain it. Then after the next 20 start ups it re-configures the input values? Is this worth trying at all, or would a CTS (coolant temp sensor, I'm guessing) maybe faulty causing this to happen only at cold start ups?

Click to expand...

I normally use a scanner that sends instructions to the PCM to perform a reset. Draining the capacitors to erase memory seems to work for some people. As far as I know it's an undocumented reset procedure.

Resets are usually performed e.g. after a tune up, repairs, maintenance, cleaning parts, etc. so the PCM is able to relearn new values. When new i/o values become prevalent, the PCM needs to conditionally recompute those values. Primarily for emission purposes.

The relearning process isn't based only on engine starting, re-learning becomes conditional in order to complete test procedures.

A reset may be worth trying.

I might first write down any stored codes, which are erased along with freeze frame data (snapshot of vehicle data at the time of a fault).

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You can also try disconnecting the CTS plug connector before and/or just after a cold start then listen to the engine, no significant changes might indicate a faulty CTS.

There are also various tests of sensors and their connections (while installed and removed from the wiring harness) found in service manuals, on-line videos and instructions found from forums.

https://www.youtube.com/results?search_query=testing+coolant+temperature+sensor

Reset to fix start-up and running problems.

Resetting the PCM normally doesn't cure a problem. Usually there is something else that caused the problem in the first place. If a reset does effect the problem then it is likely the problem will occur again, before and/or after emissions and other tests are performed.

The origin of a problem is usually mechanical, electrical, faulty sensor, fuel delivery, ignition, air intake, charging system, battery, etc.

Thanks mud, yeah it had one of those JET piggyback ecu things on it and I thought it was that. After tune up and removal of that it seemed better but still stumbled off idle when cold. If u give more than half throttle it clears up fine. What scan tool can do a hard reset on the PCM?

little squirt said:

Thanks mud, yeah it had one of those JET piggyback ecu things on it and I thought it was that. After tune up and removal of that it seemed better but still stumbled off idle when cold. If u give more than half throttle it clears up fine. What scan tool can do a hard reset on the PCM?

Click to expand...

I believe all of them can, except ones that only read and display codes and haven't included a reset button.

little squirt said:

Thanks mud, yeah it had one of those JET piggyback ecu things on it and I thought it was that. After tune up and removal of that it seemed better but still stumbled off idle when cold. If u give more than half throttle it clears up fine. What scan tool can do a hard reset on the PCM?

Click to expand...

Scanners with more features usually cost more $$$.

I would definitely perform a reset after removing a Jet performance module or chip, most likely a reset was performed just prior or after it was installed.

What is the model number?

Are you planning on selling or re-installing?

little squirt said:

Is there specific ohm values I should be looking for to tell if its bad or not?

Click to expand...

CTS temp vs. ohm table: http://www.wranglerforum.com/attachment.php?attachmentid=1147474&stc=1&d=1401239563

MAP test:

http://troubleshootmyvehicle.com/jeep/4.0L/how-to-test-the-map-sensor-1
http://www.fourwheeler.com/how-to/engine/1401-driveway-diagnostics-map-sensor/
http://www.cherokeetalk.com/forum/f64/map-sensor-test-procedure-1261/

Muddzz said:

Scanners with more features usually cost more $$$.

I would definitely perform a reset after removing a Jet performance module or chip, most likely a reset was performed just prior or after it was installed.

What is the model number?

Are you planning on selling or re-installing?

Click to expand...

I'm not sure, its laying on the shelf in the garage right now. I'll see if my local shop can do a hard reset and see if that changes anything.