Stalls / Throttle Cuts, Keeps Power, No Code

[3DRS]

Hi all, new here. I've got a 99 4.0 Auto 3 Speed that's been acting up on me for the past 2 months. While I'm driving, under any load condition (A/C, heater, headlights, etc on or off) it wants to cut power or fuel to the engine. I have to "save" it by slamming on the gas to prevent a stall, and it does fully stall out if no gas is given. Speed and tach drop, but I can't tell if it's electrical or just result of speed/rpm loss due to no gas. All other electrical systems remain working, and I get no CEL or codes. Only time I get a CEL is when it loses throttle intermittently. CEL shuts off immediately after normal throttle returns to the engine. It is worse after longer operating times/higher temperatures, but at lower temps I can trigger it by driving over heavier bumps or potholes, of which we have many. (ODOT...)

I've thrown $200 in parts and a shop visit at this, and the shop said they couldn't induce a stall or reproduce any issues when they drove it.

Things I've tried from reading forum posts:

Replaced Crankshaft Position Sensor (Walker)
Cleaned Throttle Body
Throttle Position Sensor (NAPA Echlin)
Upstream O2 Sensor (NAPA Echlin)
Vacuum system was checked by the shop, including the MAP sensor, no issues or leaks found
Alternator/belts are less than a year old
Air Charge Temp Sensor (NAPA Echlin)
Checked, cleaned, and reset all system grounds
Cleaned battery terminals and fully recharged battery
Pulling, shaking, and otherwise jiggling any wire I could get my hands on in the engine while running to listen for any drops or idle roughness, found nothing.


Things I'm suspecting/Leaning towards:

Fuel pump assembly, wiring or similar. Gas gauge was acting funny earlier this month, I would watch it surf from F to E then settle at 1/2 or 3/4 only about 2 miles away from a full refill, but I haven't seen that happen for a week. Fixes itself when power cycled.
Camshaft position sensor under the distributor, though I've seen that this would normally cause a code and inability to start, neither of which I have had.

Please, if you have any advice, let me know. This is driving me crazy because it's still driveable but lurches worse than a rodeo bull when accelerating both when straight and through turns if my touch on the gas isn't lighter than a feather.

 

[Chris]

Hi all, new here. I've got a 99 4.0 Auto 3 Speed that's been acting up on me for the past 2 months. While I'm driving, under any load condition (A/C, heater, headlights, etc on or off) it wants to cut power or fuel to the engine. I have to "save" it by slamming on the gas to prevent a stall, and it does fully stall out if no gas is given. Speed and tach drop, but I can't tell if it's electrical or just result of speed/rpm loss due to no gas. All other electrical systems remain working, and I get no CEL or codes. Only time I get a CEL is when it loses throttle intermittently. CEL shuts off immediately after normal throttle returns to the engine. It is worse after longer operating times/higher temperatures, but at lower temps I can trigger it by driving over heavier bumps or potholes, of which we have many. (ODOT...)

I've thrown $200 in parts and a shop visit at this, and the shop said they couldn't induce a stall or reproduce any issues when they drove it.

Things I've tried from reading forum posts:

Replaced Crankshaft Position Sensor (Walker)
Cleaned Throttle Body
Throttle Position Sensor (NAPA Echlin)
Upstream O2 Sensor (NAPA Echlin)
Vacuum system was checked by the shop, including the MAP sensor, no issues or leaks found
Alternator/belts are less than a year old
Air Charge Temp Sensor (NAPA Echlin)
Checked, cleaned, and reset all system grounds
Cleaned battery terminals and fully recharged battery
Pulling, shaking, and otherwise jiggling any wire I could get my hands on in the engine while running to listen for any drops or idle roughness, found nothing.


Things I'm suspecting/Leaning towards:

Fuel pump assembly, wiring or similar. Gas gauge was acting funny earlier this month, I would watch it surf from F to E then settle at 1/2 or 3/4 only about 2 miles away from a full refill, but I haven't seen that happen for a week. Fixes itself when power cycled.
Camshaft position sensor under the distributor, though I've seen that this would normally cause a code and inability to start, neither of which I have had.

Please, if you have any advice, let me know. This is driving me crazy because it's still driveable but lurches worse than a rodeo bull when accelerating both when straight and through turns if my touch on the gas isn't lighter than a feather.

You've already done quite a bit of troubleshooting and have some key components narrowed down, which is a great start. Here are a few suggestions you could consider next:

Fuel Pump and Fuel Pressure Regulator: Since you mentioned the gas gauge acting funny, it could indicate an issue with the fuel pump or the wiring to it. A weak or failing fuel pump may not be maintaining proper pressure, causing the engine to hesitate under load. You might want to check the fuel pressure to see if it’s within the acceptable range.

Electrical Connections: Given that hitting bumps can trigger the issue, it may suggest an intermittent electrical connection. While you've already jiggled wires, consider checking the wiring harness connectors for corrosion or loose connections, especially around the fuel pump and engine bay. Sometimes even the smallest amount of corrosion can create issues. This would be my very first suspicion. It happens more often than not and often times they can be very tricky to locate. As our vehicles age though, damage to the wiring harness is more common.

Ground Connections: You've mentioned that you've cleaned and checked the grounds, but if there’s a ground connection that's still not making good contact, that could cause random electrical issues. Make sure that all ground points are clean and securely attached.

Engine Control Module (ECM): If the problem persists and you’ve ruled out all other components, it could be worth considering if the ECM itself may be failing. This is less common, but if there's a bad solder joint or internal issue, it could cause erratic behavior without throwing codes. If you suspect this, I would strongly encourage sending your PCM to Mark (@Wranglerfix) as he can test it for you and rule out whether it is an ECM issue or not.

Other Sensors: In addition to the camshaft position sensor, consider checking the MAP sensor again, or any other sensors that monitor the engine's performance during load. Sometimes intermittent sensor failures don’t throw codes immediately but can still affect performance.

Test Drive in Controlled Conditions: If you have a friend or a shop that can help, try to recreate the conditions when the power loss happens safely. This might allow for more immediate troubleshooting to see if you can spot the problem when it occurs.

Unfortunately, intermittent issues like this can be a real challenge to diagnose. In these sort of situations it can really help to have a scanner that can read live data, that way you can see what the engine is doing when it acts up. Keep us posted on what you find!

 

[macleanflood]

Check fuel pressure. More importantly rig the gauge someplace you can watch it while you drive around a bit.

Monitor live ODB data...I'd be looking at long term and short term fuel trims.

-Mac

 

[TJ Jim]

Seems like it happens near idle rpm. Clean your idle air control and see if it helps. Like @macleanflood said,you need to know your fuel pressure. Should be close to 50psi.

 

[SkylinesSuck]

Sounds like a harness problem or maybe PCM. But probably harness. Great trouble shooting so far. Were you jiggling wires UNDER the vehicle or just the engine bay? The circuit that feeds the O2 sensors shares with the ASD relay and some other important things. I've seen that short and cause the exact same issue you have before. The harness also likes to wear where it snakes around the corner at the back of the cylinder head.

 

[3DRS]

I appreciate the responses so far. It happens typically towards lower RPMs, but can trigger on highways with bumps, and I have cleaned the IAC. The idle is good both under load and not, and the stalling only happens after I’ve begun accelerating to around 1k RPM, never in idle.


I’ve pulled wires both above and below the engine but I’m not opposed to trying again as there’s always a chance I may have missed something. Should I be looking at the back of the engine near the firewall for that ASD relay circuit?


As far as an OBD2 scanner capable of live data or fuel pressure testing, I’m a college student and don’t really have the budget to drop what looks like $200 on each of those just for diagnostics. Does anyone have recommendations for those on the cheaper side or ideas on places I could rent one from?

 

[macleanflood]

A $30 ODB Bluetooth dongle and the free Torque Lite app will do it.

-Mac

 

[ColoJeep]

I appreciate the responses so far. It happens typically towards lower RPMs, but can trigger on highways with bumps, and I have cleaned the IAC. The idle is good both under load and not, and the stalling only happens after I’ve begun accelerating to around 1k RPM, never in idle.


I’ve pulled wires both above and below the engine but I’m not opposed to trying again as there’s always a chance I may have missed something. Should I be looking at the back of the engine near the firewall for that ASD relay circuit?


As far as an OBD2 scanner capable of live data or fuel pressure testing, I’m a college student and don’t really have the budget to drop what looks like $200 on each of those just for diagnostics. Does anyone have recommendations for those on the cheaper side or ideas on places I could rent one from?

Take a look at the wiring harness to the upstream O2 sensor , My 99 would cut out and run funky and act like it was running out of gas . I changed the fuel pump and it still did it . one day I noticed the wire insulation was scrapped bare on the leads to the sensor , I taped them up and the issue disappeared.
But hey at least I had a new fuel pump in my haste .

 

[3DRS]

O2 sensor wiring harness externally looks good and does fine with the shake test, though most of these wires look to have been replaced at some point, given the volume of electrical tape.

I may have narrowed it down to something thanks to Skylines’ suggestion:
I was out here pulling wires again when I tried to shake this exact spot near the firewall and the engine completely shut off. I have video of the dash as I’m doing this testing on my own, but I can’t post it. The CEL was on and then went away as soon as the shutoff was complete. Likely culprit?

 

[3DRS]

O2 sensor wiring harness externally looks good and does fine with the shake test, though most of these wires look to have been replaced at some point, given the volume of electrical tape.

I may have narrowed it down to something thanks to Skylines’ suggestion:
I was out here pulling wires again when I tried to shake this exact spot near the firewall and the engine completely shut off. I have video of the dash as I’m doing this testing on my own, but I can’t post it. The CEL was on and then went away as soon as the shutoff was complete. Likely culprit?
View attachment 639408

Repeated 4x, 100% shutoff rate

 

[macleanflood]

If there's electrical tape and not heat shrink it's a bad repair and likely corroded. I'd remove the tape and inspect and I'd probably be using good barrel splices and heat shrink. Might need to replace sections of wire.

-Mac

 

[SkylinesSuck]

Repeated 4x, 100% shutoff rate

 

[3DRS]

This orange wire has a slice in it from wearing against the ground bolt standoff at the back of the engine. It was shorting to the ground on the engine and causing my stalls. Not sure what it goes to but the rest of the wiring looks good. @SkylinesSuck

 

[3DRS]

This orange wire has a slice in it from wearing against the ground bolt standoff at the back of the engine. It was shorting to the ground on the engine and causing my stalls. Not sure what it goes to but the rest of the wiring looks good. @SkylinesSuck

Hit post before I meant to. You are a lifesaver. I appreciate all the responses and the willingness to help. Time to practice my wiring skills.

 

[lBasket]

Hit post before I meant to. You are a lifesaver. I appreciate all the responses and the willingness to help. Time to practice my wiring skills.

Buy heat shrink and uninsulated butt connectors, I bought the combos and they're wayy bigger and uglier.

 

[3DRS]

Photo didn’t make it either lol

 

[be77solo]

I'd check that red/green wire too while in there....

 

[hear]

Has all the earmarks of a crank sensor issue, but sounds like specifically it's the wiring. I did notice that you didn't mention the cam sensor. Those two have nearly identical operation, and the wires both run in that same section of loom. There are some continuity tests you can run but ultimately you're probably going to need to open up that section of harness and just get to inspectin'.