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Backfiring in the Intake Manifold

M

mrpeepers

NAXJA Forum User
Location
Phoenix, AZ
Hi All,

I've been sitting on my hands the past couple weeks a bit stumped.

90 Renix 4.0L
I have replaced plugs/cap/rotor

Jeep cranks longer to start, always starts. Now, it hesitates and loses power between 20-35 mph, when you give it gas.

I'm stuck, I don't have a fuel pressure gauge, vacuum gauge, etc.

Recently failed emissions for NOX. I figure I have a bad EGR, so I just blocked it off. Would a bad CAT Convertor cause intake manifold backfiring?

Things ran perfect till I changed fuel filter. It is the correct flow direction. FRAM. Bad Fuel Pump?

Any ideas?
 
yes there is a correct flow direction but i dont think you can install it backwards.

if this all started after you replaced parts, you need to check your work and look to make sure you didnt break or knock anything off.
 
Backfiring into the intake means the engine is running lean. The filter should be clearly marked with the direction of flow.

Everytime I have experienced this backfire I was replacing a fuel pump shortly thereafter.
Good Luck.
 
I double checked fuel filter and even replaced that little hose on the engine end of the filter.

the fuel pump hisses. I've been reading on here its pretty common and didn't necessarily mean it was bad.

What psi should fuel pump be sending fuel to the rail? would a bad fuel pressure regulator lead to the same issue?
 
mrpeepers said:
Hi All,

I've been sitting on my hands the past couple weeks a bit stumped.

90 Renix 4.0L
I have replaced plugs/cap/rotor

Jeep cranks longer to start, always starts. Now, it hesitates and loses power between 20-35 mph, when you give it gas.

I'm stuck, I don't have a fuel pressure gauge, vacuum gauge, etc.

Recently failed emissions for NOX. I figure I have a bad EGR, so I just blocked it off. Would a bad CAT Convertor cause intake manifold backfiring?

Things ran perfect till I changed fuel filter. It is the correct flow direction. FRAM. Bad Fuel Pump?

Any ideas?
Click to expand...

Wow. This sounds so close to what I'm going through.:idea:
1990 XJ, 4.0 AW4, 198,000 miles. Been running mostly fine except for a little stumble a throttle tip in. Until my daughter was driving it to college and halfway there it started losing power on hills. Ultimately it overheated before we finished the drive. Once I towed it home, and let it sit for a while, replaced the following, radiator and hoses, TPS, plugs wires rotor and cap, and fuel filter. No change. Went back and checked compression, bad headgasket. Replaced it. Cleaned things up as I went, throttle body is spotless, replaced the injectors (used though) and new O rings. No change.

This evening, swapped the MAP sensor, checked the IAC, unplugged the EGR. No change. Oddly, I hadn't noticed before last night, that there is a smell of gas in the exhaust, but not like raw fuel (like I used to get with my Daytona Turbos...). I was thinking I might swap the distributor (and cam sensor) out of my parts truck. But maybe I'll change the fuel pump first.:wave:

Driving me nuts!:gee:
 
slowjeep said:
any status on either one of you
Click to expand...

i haven't had the time to replace the pump. I want to test the fuel rail before so. Anyone know the standard fuel pressure off the rail?

Longer Crank times, hesitation, backfiring and loss of power between 20-45 mph when gassing it, and gassy exhaust smell.

it does idle fine at 500 rpms, and has a slight stutter at idle, but without the crank and loss of power you wouldn't know anything was wrong.


btw heres a guide if you needed one.
http://www.allpar.com/SUVs/jeep/fuel-pump.html
 
fuel pressure on the fuel rail should read 31psi with the vacuum line connected to the regulator and 39psi without the vacuum line connected to the regulator.
 
I need to borrow or buy a fuel pressure gauge. My friend's doesn't read over 20...

As for progress, changed the Map and reset the ECM, no change. pulled th vac line off the EGR and no change, verified the IAC was working too.

After finding this thread last night, I'll pull the pump from my parts rig before spending the bucks for a new pump.

Its nice to have a parts rig... And its been a while since I looked at Allpar.
 
XJ is running, heres the update.

Wasn't the CPS, what a pain...

Wasn't the Cam Sensor either...

I even swapped the Ignition module...

Had I not listened to friends, I would have pulled the fuel pump and found this much earlier...!!!1

HELLO!!!! The little 3" length of hose INSIDE the tank is ROTTEN!!! I pulled it off the fittings, clamps and all, by HAND!!!!
XJFuel005.jpg

And the clamp wasn't even fully on the fitting...

So while I had fuel at the rail, it wasn't consistant or steady. Replaced it with some fuel injection hose and started it. Settled right down to a nice smooth idle and stayed there.

Took it for a drive and it runs so much better now. I'll see how it runs by the end of the week. I feel so much better now... :gee:
 
Thanks! One of my friends originally thought it might be a fuel delivery issue. But then when we found the headgasket blown, we forgot all about fuel. Between my being sick, having minor surgery and a bit of don't give damn attitude, this took way longer to figure out...
 
I had that same issue, I had the xj parked for about 2months and replaced the IAC, TPS, and CPS before noticing it was the line was almost completely off. But now it is back on the road and NO problems!!!!!
 
That little rubber hose is a PITA $.40 leaves you stranded.
When mine went it died like it was out of gas but, had plenty in the tank.

I have a feeling this will become a common problem as the RENIX XJ's are coming of age.

Does anybody know about newer pump assy.s are setup like RENIXs? 4" rubber fuel line.
 
Did the original poster of the thread find his problem? I know i am going to check my pump tomorrow now to check for that fuel hose! thanks for the info.
 
I have similar problem in my 98 XJ. Rough idle and has dead spot in pedal around same mph, and almost dies but never actually dies. Replaced the tps, new cat muffler and tailpipe. Maybe its the 02? let me know how you make out, maybe the fuel problem is it too, gotta check tomorrow
 
easiest way to find out is take the fuel pump out and check the line, I don't about other years, but on my '90 the pump was accessible without removing the tank. And replace that hose, it' won't cost you more than $2.00. When I would check the fuel pressure from the valve(no gauge) after shutting the ignition off I had very little or no fuel in the system, at first I thought that was normal, until a fresh mind came to my rescue. Don't go crazy buying parts!!!!
 
been a long time since i looked at the thread.

i haven't changed my problem. I thought it was bad EGR valve and still do with the failed NOX emissions test. I don't know how to test it, and couldn't find a vacuum problem. I suppose that little hose may be worth looking at.

same symptoms however, intake manifold pops from 25-40 under load. never when it's light on the pedal just when you gas it or gun it. mileage is nothing to write home about. But it still seems to be in the 15-20 city range.
 
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