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  #1  
Old August 26th, 2012, 07:18
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Backfire out intake/no power/bogs down

1987 XJ, 4.0L, Renix, 4X4, AW4

Spent a lot of time on this already. Refreshed grounds, new TPS, new distributor cap, rotor, plugs, cleaned connectors etc. Jeep runs fine when cool. Starts fine. Fuel pump is noisy but when cold, every seems ok-can accelerate normally. When Jeep gets hot, I go to a no power condition. Can barely get engine to rev past idle. If I increase throttle, it backfires out intake and eventually dies.

We've had some good weather and it was running great every day. Yesterday was pretty hot and it was running fine and then suddenly it had no power and it would backfire through the intake if I stepped on it. Limped home and took a look at throttle body. Very clean etc. When I moved the throttle manually it revs up and then starts bogging down. If I move it quickly, it will backfire. I'm thinking it's a hot-wire issue. I've re-done the grounds, re-did negative battery cable to block, cleaned up dipstick grounds, cleaned up block to firewall ground etc. I had routed all of the wiring away from heat spots etc. It has a new TPS sensor. I just re-did rear shoes, drums and wheel cylinders, front ball joints and u-joint, new e-brake, so I've got money in it now.

Thinking it might be coolant temp sensor, knock sensor or O2 sensor but not sure. Did the fuel pressure at the rail and it goes to zero when I move the throttle plate open but could it be the ECU telling it to back off on pumping if the sensors are saying the engine is cold? Kind of stumped at this point.

Willing to change fuel pump but don't want to rule out sensor issue first.

Thanks!
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  #2  
Old August 26th, 2012, 07:34
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Re: Backfire out intake/no power/bogs down

I don't beleive ECU ever tells fuel pump to "back off on pumping"... it expects pressure to be there when it tells the injectors to fire...
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  #3  
Old August 26th, 2012, 10:48
Monster Mopar Monster Mopar is offline
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Re: Backfire out intake/no power/bogs down

Sounds like you have a fuel pressure issue to me. The fuel pump is not controlled by the ecu. It's either on or off. the pressure is regulated by the fuel pressure regulator at the front of the fuel rail.

It's either the fpr, the pump, or there is a rubber hose connecting the pump to the fuel sending unit that is known for coming off.

The hose came off on mine partially so it would run but had no power and back fired out the intake as you describe.
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  #4  
Old August 26th, 2012, 15:06
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Re: Backfire out intake/no power/bogs down

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Originally Posted by Monster Mopar View Post
It's either the fpr, the pump, or there is a rubber hose connecting the pump to the fuel sending unit that is known for coming off.
I agree these would be the prime suspects.

You should have about 31 psi of fuel pressure with the vacuum line connected to the FPR and about 39 psi with it off.
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  #5  
Old August 26th, 2012, 15:21
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Re: Backfire out intake/no power/bogs down

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Originally Posted by Saudade View Post
I agree these would be the prime suspects.

You should have about 31 psi of fuel pressure with the vacuum line connected to the FPR and about 39 psi with it off.
Thanks for the replies. When I first start it up, I have the right fuel pressure. I drove it today, started out ok but then about 10 minutes in, it had no throttle response other than backfire/bog etc. Limped home and I could here the fuel pump sounding like morse code in the back (noise off and on). Fuel pump pressure erratic and would go to zero if I moved the throttle to more than 1/4 of the way.
I'm going to take the plunge and order a new pump. RockAuto has a carter for $70. I've heard the airtex are not good. Any thoughts?
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  #6  
Old August 26th, 2012, 15:31
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Re: Backfire out intake/no power/bogs down

Fortunately, you can access the pump without dropping the tank. Just make sure you're low on fuel.

If you're daring, you can pull the pump assembly to check the hose before you buy parts. Just be careful with the gasket that seals the flange.
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Old August 26th, 2012, 15:52
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Re: Backfire out intake/no power/bogs down

I would be more likely to believe you have a cross fire on spark plug wires. Backfire is a symptom of the spark being at the wrong time. You can have an engine run so hot that you get preignition but you should see a significant rise in coolant temp.
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  #8  
Old August 26th, 2012, 16:23
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Re: Backfire out intake/no power/bogs down

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Originally Posted by old_man View Post
I would be more likely to believe you have a cross fire on spark plug wires. Backfire is a symptom of the spark being at the wrong time. You can have an engine run so hot that you get preignition but you should see a significant rise in coolant temp.
Coolant gauge never seems to go much past 100. I assume that's Celsius since it has 40, 100 and 125 and it's Canadian. 100 would be 212 F. I know stock gauges aren't always accurate but it does move. I'm thinking that the fuel injectors are opening and the ECU is trying to adjust the pulse width assuming there is fuel at the ready. If the pump is erratic, maybe you have a fuel/no fuel issue combined with heavy throttle angle making the ECU request more fuel than needed and hence the backfire? Don't know, just guessing. I had lots of transmission issues due to the speed sensor getting too hot and grounding out on the trans case.

This Jeep does seem to run much hotter than anything I've had before. The electric fan comes on and off so I think it's aware of the heat. Since the TPS and CPS are good, new cap, rotor, plugs and wires, I don't think it's a spark issue.
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Old August 26th, 2012, 17:22
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Matthew Currie Matthew Currie is offline
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Re: Backfire out intake/no power/bogs down

Long ago My 87 had a problem with the fuel pump wiring - where the harness came out, it got knocked by the suspension, and had a tendency to go dead on railroad crossings because of this. Before you go too deep into the fuel pump you should make sure there's not a poor piece of wire, pehaps a broken conductor hiding behind good insulation, somewhere.
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  #10  
Old August 27th, 2012, 05:11
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Re: Backfire out intake/no power/bogs down

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Originally Posted by Matthew Currie View Post
Long ago My 87 had a problem with the fuel pump wiring - where the harness came out, it got knocked by the suspension, and had a tendency to go dead on railroad crossings because of this. Before you go too deep into the fuel pump you should make sure there's not a poor piece of wire, pehaps a broken conductor hiding behind good insulation, somewhere.
I'm going to look at this but I'm not sure if it would make sense since it is doing the rough running when it's sitting in my driveway. I haven't checked voltage at the connector when it's running bad but it's voltage is normal when not running. I think this will be the last thing I'll check before I get a new pump.

Anyone know if you need to replace the fuel pump with a similar pump? I saw in another post that you have to switch a bosch for a bosch, walpro for a walpro? Not sure why that would be unless the fittings are different.
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Old August 27th, 2012, 06:24
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Re: Backfire out intake/no power/bogs down

A burnt intake valve can cause back fire. I would pull the plugs and take a look at them, then do a wet/dry compression test.
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  #12  
Old August 30th, 2012, 18:19
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Re: Backfire out intake/no power/bogs down

Put a new Carter fuel pump in. The pump I removed was a Bosch but I didn't know that before I ordered the Carter. There is a slight incompatibilty with the wiring and the rubber connectors to the assembly but I was able to get it all together and working. Seems good but I haven't had time today to give it a good shakedown. Only took about 15 minutes to get the pump out so I would recommend for anyone replacing their pumps to pull it first and then see what you need to order. Jeep started right up after two switch on/off to prime the rail.

Spark plugs, wire, rotor, distributor cap were all changed in June so I think they should be good.
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  #13  
Old August 30th, 2012, 19:32
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Re: Backfire out intake/no power/bogs down

If you didn't replace the O-ring and locking ring on the fuel tank when you pulled it, you should. Spectra Premium part number LO1 (that's a letter O, not a number 0) is like 7 bucks at every parts store. I'd get it and keep it on hand in case you have to go in there again or it starts leaking.

For future reference the fuel pump on a RENIX or early OBD1 has only 3 settings - full pressure, partial pressure, and off. Partial pressure is a low voltage operating mode that only exists to keep pump noise to a minimum in the cabin during normal operation. The ECU turns the pump on full pressure for a few seconds at startup to ensure the fuel rail is full, then backs off to partial pressure and leaves it there until the jeep is shut off. If the jeep ever develops a problem where it'll start and run for a few seconds then quit every time, you know that either the relay or the ballast resistor for it (mounted on the left inner fender) is at fault - or the associated wiring.

Bypassing the resistor (which makes partial pressure the same as full pressure) may somewhat reduce the lifetime of the fuel pump but I doubt it, it was designed to operate on 12V in the first place. It does remove a known weak point of the ballast resistor corroding, mine was in pieces when I bought the MJ so I simply removed it and used a splice terminal to bypass it. Haven't had any issues yet on the stock 240k mile 21 year old fuel pump.
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Last edited by kastein; August 30th, 2012 at 19:38.
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  #14  
Old August 30th, 2012, 20:01
Marcelo73 Marcelo73 is offline
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Re: Backfire out intake/no power/bogs down

I have a 98 but with similar problem. I took it to the taller. First tought the problem was in the gas pump but finally was the O2 sensor. It was burned out.
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Old August 30th, 2012, 21:48
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Re: Backfire out intake/no power/bogs down

Quote:
Originally Posted by old_man View Post
A burnt intake valve can cause back fire. I would pull the plugs and take a look at them, then do a wet/dry compression test.
If it were not for him seeing a full supply issue already at the fuel rail sometimes, I would agree on the burnt valve not sealing when it gets too hot, and add the possibility of temp sensor, MAT or CTS malfunction, or MAP sensor, working one minute then not the next. I would also look for a busted Cat or muffler, that chokes off the exhaust when it gets too hot, and moves.

I remember vapor lock in fuel lines when they got too hot, but they were not under pressure. Perhaps there is bad hose in the fuel line that comes and goes with heat?

Could some junk be randomly clogging the vacuum line to the Map sensor that is temperature sensitive. Junk in the throttle body at that connection.

Thing to do is be ready to run volt meter tests everywhere when it malfunctions!!!!!!!
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