Misfire at WOT

[trippled]

These values look normal.

 

[dan1977p]

Prong A (not running) 5.05 volts
prong B (not running) 4.34 volts
Prong C (not running) 0.02 volts

A (running) 5.06 volts
b (running) 1.63 volts
C (running) 0.04 volts

What was done to change those values from before? Did it help any?

 

[Wiser]

What was done to change those values from before? Did it help any?

The top numbers are with the jeep not running but the key in the ON position. Bottom numbers are the the jeep running and warmed up. I did nothing for the numbers to change slightly.

 

[Ecomike]

Ehall mentioned the coil too and if it's sending too much juice it could burn your plugs too.

I have never ever heard of a coil putting out too much juice and burning up plugs. Not possible.

Plugs get burned up if they are too hot for the engine design. Too hot means they do not cool as fast as stock plugs and the ceramic insulator super heats over time. Or running too lean, too little fuel will burn the plugs, even OEM stock plugs but hot plugs would fry even faster.

 

[Ecomike]

Is the check engine light bulb burned out?

 

[Ecomike]

We should have caught this on your post

"Put fuel rail on a gauge and its 40psi idle, 41-43 at WOT with no load. It does fluctuate between 41-43, is this normal

NO!! That is not normal!!!!

for 87-90, and I think it is the same on 91-95?"

From and old site early expert on Renix

http://www.lunghd.com/Tech_Articles/Fuel_System/Fuel_Diagnostics.htm

I rechecked your first post, you may have a fuel pressure control problem?

System fuel should be 214 kPa (31 psi) with the vacuum line connected to the regulator and 269 kPa (39 psi) with the vacuum line disconnected. CAUTION: Some fuel may be discharged when connecting fuel gauge to fuel rail.

1. Connect a 0-414 kPa (0-60 psi) fuel pressure gauge to test port pressure fitting on fuel rail (Fig. 7).
2. Remove vacuum line from pressure regulator.
3. Start the vehicle.
4. Note gauge reading. With vacuum line disconnected, fuel pressure should be approximately 269 kPa (39 psi).
5. Connect vacuum line to pressure regulator. Note gauge reading. Fuel pressure should be approximately 214 kPa (31 psi).
6. If fuel pressure is not approximately 55-69 kPa (8-10 psi) higher with vacuum line removed from regulator, inspect pressure regulator vacuum line for leaks, kinks or blockage. CAUTION: Fuel pressure will rise to as much as 655 kPa (95 psi) when the fuel return tine is pinched shut, shut engine down immediately after pinching oft fuel return line.
7. If fuel pressure is low, momentarily pinch shut the hose section of the fuel return line. If fuel pressure remains low, inspect the fuel supply line, fuel filter, and fuel rail inlet for blockage. If fuel pressure rises replace fuel pressure regulator.
8. If fuel pressure is above specifications, inspect the fuel return line for kinks and blockage.

Capacity Test

1. Remove the cap from the pressure test port in the fuel rail.
2. Connect a 0-414 kPa (0-60 psi) fuel pressure gauge to the pressure fitting on the fuel rail (Fig. 7).
3. Start the vehicle. Pressure should be approxi- mately 214 kPa (31 psi) with the vacuum hose connected to the pressure regulator and 269 kPa (39 psi) with the vacuum hose removed from the pressure regulator.
4. If the pressure is not to specification, check the following before replacing the fuel pressure regulator:

• 4a - Inspect the fuel supply and return lines/hoses for kinks or restricting bends
• 4b - Check the fuel pump flow rate. A good fuel pump will deliver at least 1 liter of fuel per minute with the fuel return line pinched off. If the fuel pump does not pump adequately, then inspect the fuel system for a plugged fuel filter or fuel pump inlet filter (sock). Fuel pump flow rate can be done by connecting one end of an old A/C gauge hose to the fuel test port on the fuel rail and inserting the other end of the hose into a container of at least 1 liter capacity. Run the fuel pump by installing a jumper wire into diagnostic connector terminals D1-5 and D1-6. Be sure to pinch off the fuel return line or most of the fuel will be returned to the fuel tank.

The RENIX 4.0L fuel pressure regulator is not adjustable.
MPI Fuel Pressure Leak Down Test with Engine Off

If abnormally long periods of cranking are required to restart a hot engine after the vehicle has been shut down for a short period of time, fuel pressure may be bleeding past the fuel pressure regulator or the check valve in the outlet end of the fuel pump.

1. With the engine off, connect an accurate 0-689 kPa (0-100 psi) fuel gauge to the pressure test port fitting on the fuel rail.
2. Start the vehicle and let engine idle. Check fuel pressure reading on gauge. Fuel pressure should be within specifications. Refer to MPI System Fuel Pressure Test.
3. Shut engine off. Note fuel pressure reading on gauge. Leave fuel pressure gauge connected. Allow engine to set for 30 minutes and then compare fuel pressure reading on gauge to reading taken when engine was shut down. A pressure drop up to 138 kPa (20 psi) to a range of 131-269 kPa (19-39 psi) is within specifications.
4. If fuel pressure drop is within specifications, the fuel pump outlet check valve and the fuel pressure regulator are both operating normally.
5. If fuel pressure drop is greater than 138 kPa (20 psi), restart the vehicle and let engine idle. CAUTION: Fuel pressure will rise to as much as 95 psi when the fuel return line is pinched shut, shut engine down immediately after pinching off fuel return line.
6. Momentarily pinch shut the hose section of the fuel return line and shut engine off. Note pressure reading on gauge. Allow engine to set for 30 minutes. Compare fuel pressure reading to reading taken when engine was shut down. If fuel pressure drops approximately 138 kPa (20 psi), replace the fuel pressure regulator. If fuel pressure has dropped considerably more than 138 kPa (20 psi) fuel pressure is bleeding off past the outlet check valve in pump and the fuel pump must be replaced.

 

[Ecomike]

"1991-95 MPI systems operate at 31-39 PSI." found on another site for jeeps, so the vacuum line or the fuel pressure regulator, or both seem to be bad.

 

[Wiser]

Is the check engine light bulb burned out?

No, it does work. I cleaned the throttle body early and it threw a code 25 when I first started it up. I reset the computer and the code is gone.

"1991-95 MPI systems operate at 31-39 PSI." found on another site for jeeps, so the vacuum line or the fuel pressure regulator, or both seem to be bad.

I swapped out the fuel pressure regulator with a known good one and that had bumped my pressure up about 1-2psi initially but stopped at 43 PSI. The small vaccum line that runs from the fuel regulator to the manifold vacuum port does seem kinda loose fitting. It looks as though the owner before me had broken the line and put it back together with a larger one to make it work.

 

[Wiser]

We should have caught this on your post

"Put fuel rail on a gauge and its 40psi idle, 41-43 at WOT with no load. It does fluctuate between 41-43, is this normal

NO!! That is not normal!!!!

for 87-90, and I think it is the same on 91-95?"

From and old site early expert on Renix

http://www.lunghd.com/Tech_Articles/Fuel_System/Fuel_Diagnostics.htm

I rechecked your first post, you may have a fuel pressure control problem?

System fuel should be 214 kPa (31 psi) with the vacuum line connected to the regulator and 269 kPa (39 psi) with the vacuum line disconnected. CAUTION: Some fuel may be discharged when connecting fuel gauge to fuel rail.

1. Connect a 0-414 kPa (0-60 psi) fuel pressure gauge to test port pressure fitting on fuel rail (Fig. 7).
2. Remove vacuum line from pressure regulator.
3. Start the vehicle.
4. Note gauge reading. With vacuum line disconnected, fuel pressure should be approximately 269 kPa (39 psi).
5. Connect vacuum line to pressure regulator. Note gauge reading. Fuel pressure should be approximately 214 kPa (31 psi).
6. If fuel pressure is not approximately 55-69 kPa (8-10 psi) higher with vacuum line removed from regulator, inspect pressure regulator vacuum line for leaks, kinks or blockage. CAUTION: Fuel pressure will rise to as much as 655 kPa (95 psi) when the fuel return tine is pinched shut, shut engine down immediately after pinching oft fuel return line.
7. If fuel pressure is low, momentarily pinch shut the hose section of the fuel return line. If fuel pressure remains low, inspect the fuel supply line, fuel filter, and fuel rail inlet for blockage. If fuel pressure rises replace fuel pressure regulator.
8. If fuel pressure is above specifications, inspect the fuel return line for kinks and blockage.

Capacity Test

1. Remove the cap from the pressure test port in the fuel rail.
2. Connect a 0-414 kPa (0-60 psi) fuel pressure gauge to the pressure fitting on the fuel rail (Fig. 7).
3. Start the vehicle. Pressure should be approxi- mately 214 kPa (31 psi) with the vacuum hose connected to the pressure regulator and 269 kPa (39 psi) with the vacuum hose removed from the pressure regulator.
4. If the pressure is not to specification, check the following before replacing the fuel pressure regulator:

• 4a - Inspect the fuel supply and return lines/hoses for kinks or restricting bends
• 4b - Check the fuel pump flow rate. A good fuel pump will deliver at least 1 liter of fuel per minute with the fuel return line pinched off. If the fuel pump does not pump adequately, then inspect the fuel system for a plugged fuel filter or fuel pump inlet filter (sock). Fuel pump flow rate can be done by connecting one end of an old A/C gauge hose to the fuel test port on the fuel rail and inserting the other end of the hose into a container of at least 1 liter capacity. Run the fuel pump by installing a jumper wire into diagnostic connector terminals D1-5 and D1-6. Be sure to pinch off the fuel return line or most of the fuel will be returned to the fuel tank.

The RENIX 4.0L fuel pressure regulator is not adjustable.
MPI Fuel Pressure Leak Down Test with Engine Off

If abnormally long periods of cranking are required to restart a hot engine after the vehicle has been shut down for a short period of time, fuel pressure may be bleeding past the fuel pressure regulator or the check valve in the outlet end of the fuel pump.

1. With the engine off, connect an accurate 0-689 kPa (0-100 psi) fuel gauge to the pressure test port fitting on the fuel rail.
2. Start the vehicle and let engine idle. Check fuel pressure reading on gauge. Fuel pressure should be within specifications. Refer to MPI System Fuel Pressure Test.
3. Shut engine off. Note fuel pressure reading on gauge. Leave fuel pressure gauge connected. Allow engine to set for 30 minutes and then compare fuel pressure reading on gauge to reading taken when engine was shut down. A pressure drop up to 138 kPa (20 psi) to a range of 131-269 kPa (19-39 psi) is within specifications.
4. If fuel pressure drop is within specifications, the fuel pump outlet check valve and the fuel pressure regulator are both operating normally.
5. If fuel pressure drop is greater than 138 kPa (20 psi), restart the vehicle and let engine idle. CAUTION: Fuel pressure will rise to as much as 95 psi when the fuel return line is pinched shut, shut engine down immediately after pinching off fuel return line.
6. Momentarily pinch shut the hose section of the fuel return line and shut engine off. Note pressure reading on gauge. Allow engine to set for 30 minutes. Compare fuel pressure reading to reading taken when engine was shut down. If fuel pressure drops approximately 138 kPa (20 psi), replace the fuel pressure regulator. If fuel pressure has dropped considerably more than 138 kPa (20 psi) fuel pressure is bleeding off past the outlet check valve in pump and the fuel pump must be replaced.

It does have a long crank time to start it. I'm pretty sure the check valve is bad. I was thinking of swapping in a new pump for good measure. After all, it looks to be the 25 yr old OE Pump. Another note too was I was driving home Wednesday and went to get some gas and I had a bunch of pressure built up in the tank. Was about 100* outside and my jeep was running about 215*, a little warmer than usual. Wonder if that has any thing to do with a failing pump?

 

[Ecomike]

Fix this FIRST!!!!!

"The small vaccum line that runs from the fuel regulator to the manifold vacuum port does seem kinda loose fitting. It looks as though the owner before me had broken the line and put it back together with a larger one to make it work."

Are you 100% sure the FPR you used was and is good?

THEN look for crimped or damaged return fuel line to the gas tank!!!!

 

[Ecomike]

It does have a long crank time to start it. I'm pretty sure the check valve is bad. I was thinking of swapping in a new pump for good measure. After all, it looks to be the 25 yr old OE Pump. Another note too was I was driving home Wednesday and went to get some gas and I had a bunch of pressure built up in the tank. Was about 100* outside and my jeep was running about 215*, a little warmer than usual. Wonder if that has any thing to do with a failing pump?

Long crank time is important, but I doubt it is the fuel pump, check valve or gas tank or vent issues causing the current problem, quit the opposite. Stay focused!!! A long crank time was mentioned as a low pressure, or pressure drop due to not holding pressure when the engine is off, your gauges are telling that the problem is too much pressure, not too little. I do not think there is a check valve in the gas tank fuel pump on a 91? If there is, it is working, or you would have low pressure.

Do you smell gas at the exhaust???? A sign of running rich? If not, and based on the spark plugs looking like the engine was running lean, the clues do match.

 

[Wiser]

Long crank time is important, but I doubt it is the fuel pump, check valve or gas tank or vent issues causing the current problem. Stay focused!!! A long crank time was mentioned as low pressure, or pressure drop due to not holding pressure when the engine is off, you gauges are telling t=yo the problem is too much pressure, not too little.

Do you smell gas at the exhaust???? A sign of running rich?

Hahah No I don't smell gas at the exhaust, no sign of running rich. I can post a pic of how the plugs looked when I pulled them last. In the pic the order is 1(left)-6(right). The orange tinge I think is the seafoam I had ran through the motor prior to pulling them. Sorry if the pic is huge

 

[Ecomike]

Just replace the Vacuum line (next), it may have a split side too, other than being loose and not holding a steady vacuum.

If that does not work then try another FPR anyway.

Also, if fixing the fuel pressure does not completely solve the problem, the aftermarket CPS and TPS have been known to frequently be bad out of the box, and I am sure you already know they can cause your problem. They can be tested with a multimeter while the engine is running. Jumps or spikes in the output voltage from those 2 sensors are clues they are bad (TPS can be good at certain ranges and bad, or in your case very bad at WOT for instance), or spikes when you wiggle the wiring harness are clues of damaged wiring that degrades the signal to the ECU.

 

[Ecomike]

The FPR regulates fuel pressure and changes it as engine vacuum changes, like during deceleration and acceleration. The plugs are either pretty new, or were running a bit lean, but not lean enough for severe misfires like you have. I know that many folks here swear that double platinum plugs are known to misfire on the HO engines. I have run single platinum for over 10 years on 2 Renix XJ.

 

[Wiser]

Just replace the Vacuum line (next), it may have a split side too, other than being loose and not holding a steady vacuum.

If that does not work then try another FPR anyway.

Also, if fixing the fuel pressure does not completely solve the problem, the aftermarket CPS and TPS have been known to frequently be bad out of the box, and I am sure you already know they can cause your problem. They can be tested with a multimeter while the engine is running. Jumps or spikes in the output voltage from those 2 sensors are clues they are bad (TPS can be good at certain ranges and bad, or in your case very bad at WOT for instance), or spikes when you wiggle the wiring harness are clues of damaged wiring that degrades the signal to the ECU.

Going to out to replace the vacuum line. I'll report back

 

[Wiser]

Replaced the line, no change in performance

 

[Ecomike]

Replaced the line, no change in performance

I assume you mean the vacuum line? In that case make sure you can feel vacuum in the hose end at the FPR connection. What does no change in performance mean? Did you run the fuel pressure test at idle?

" Start vehicle. Pressure should be approximately 31 psi (2.1 kg/cm) with vacuum hose connected to pressure regulator and 39 psi (2.6 kg/cm) with vacuum hose removed from pressure regulator."

If yes, and the FP is still above 39 psi all the time, then I would look for damaged tubing from the FPR on the return line back to the fuel tank, that would restrict flow. Lot of off road stuff can damage and crush something like a fuel line. If none is found, I would try a new FPR.

 

[Wiser]

I assume you mean the vacuum line? In that case make sure you can fell vacuum in the hose end at the FPR connection. What does no change in performance mean? Did you run the fuel pressure test at idle and during acceleration and deceleration, and at WOT to see if the FP ever drops to about 29-31 psi and back to about 39?

If yes, and the FP is still above 39 psi all the time, then I would look for damaged tubing from the FPR on the return line back to the fuel tank, that would restrict flow. Lot of off road stuff can damage and crush something like a fuel line. If none is found, I would try a new FPR.

I haven't put a gauge on the rail yet. Performance meaning driving it around and it still misfiring at 3k rpm. I'll have to go borrow a fuel pressure gauge

 

[Wiser]

Solution: timing was off. Smog guy must have messed with the distributor to get my jeep to pass smog. Thanks for everyone's input.