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Fuel pressure questions

SanDiegoXJ

SanDiegoXJ

NAXJA Forum User
Location
San Diego, CA
So I just failed smog with ungodly high NOx on a newly rebuilt engine. 1994 Jeep Cherokee Country 4.0 Auto.

Since the rebuild, the engine has suffered from a vibration at low torque, and has had what felt like a miss at WOT. However, it idles very smooth.

I've checked for vacuum leaks with starter fluid (I know, not the best method, but it usually works). It has new plugs, wires, cap, rotor, distributor, IAC, TPS, CTS, water pump, coolant hoses, radiator is 3 years old 3 core.

The overall consensus till this point has been air getting in the system somewhere, but I just cant find it.

Then I stuck a pressure guage on the fuel rail and fired the Jeep up. Took a look, 31psi...good news. Popped the throttle a couple of times and the pressure bounced between 31 and 38...about normal for what I know. Then I slowly applied the throttle and the fuel pressure decreased from 31-30 and stayed there (at this point I could feel the vibration kick in again). As I applied a little more pressure to the throttle, the fuel pressure dropped a hair more between 29.5 and 30 psi and the vibration intensified...from there, I could continue to apply pressure but you could still feel what I think is a miss. When I let off the throttle, pressure bounces just a bit and normalizes quickly at 31psi.

Is the decrease in pressure normal? Is it indicitive of a bad pump, clogged filter, or bad regulator?

Thanks for any input.
 
One more test needed as long as you have a gauge.

If you remove the vacuum line leading to the fuel pressure regulator, the pressure should jump to 39 psi. Be sure that the vacuum line is pulling good vacuum line and if it is wet with fuel, it means a ruptured fuel pressure regulator.

31 psi at idle is perfect.
 
Note from DJ
Post you’re Smog Numbers a lot of the members can look at them and pick up clues as to what your problem might be.

I saved this link hope it helps
How do you check the Cat NAXJA 7-12-10
http://www.naxja.org/forum/showthread.php?t=1038384

Post # 2 by Bandett007
Here is a link about reading vacuum gauges. http://www.classictruckshop.com/club...ts/vac/uum.htm
Other things to look for, for a faulty cat are the following:
Cat is glowing and or piping in front of it glowing hot.
When you hit it with a closed fist/mallet it rattles like there are rocks in it.
Your exhaust smells like rotten eggs.
When you turn the truck on or off you hear the cat rattle around.
----------------
Post # 7 by cruiser54
Connect vacuum gage to the intake manifold, perhaps where the vacuum hose for your CCV system is connected. Note the vacuum reading at idle. Rev the engine to 2000 RPM and hold steady. That reading should be the same as the idle reading. If it's lower, exhaust is restriction.

As an additional test after holding at 2000 RPM, let the throttle snap shut. The vacuum should quickly rise and then very soon after drop right down to the idle reading. If the needle comes down slowly and jerkily, exhaust restriction.
 
yeah, already did that too.

31 psi with, 39 psi without

The real question is should the fuel pressure in the rail ever be lower while under load than it is at idle.

basically, what is correct fuel pressure at say 1500-2000rpm. That's where I notice it most. It's also the same point that pressure drops from 31psi to 30psi, and it's most noticeable when warm.

EDIT: Has a brand new cat too, forgot to mention that. I know it's a lean running condition...so I figure what I need to do is find out what psi the fuel rail should have under higher rpm, get an ohm meter and check the resistance on each injector plug, (injectors are 2 year old remanned mustang injectors), and check the resistance on the o2 sensor to make sure I didn't just get a bad sensor.
 
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A little bit of pressure fluctuation is normal
 
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it sounds like you're doing this testing at idle? if so, that'd be perfectly normal. as you rev it up slowly or blip the throttle lightly, the manifold vacuum will rise a little bit, which will lower the fuel pressure a few lbs. lower vacuum to the regulator=higher fuel pressure. while driving and under load the vacuum will drop, obviously increasing the fuel pressure. be interesting to check what happens while driving. maybe hook a pressure guage up to the rail, stick it under the wiper blade or something, and take it for a spin?
 
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No one has mentioned it yet so here it is. What about the O2 sensor? That can cause it to stumble if not reading right or not reading at all. And can cause other issues as well.
 
yeah, it has a new 02 sensor in it. I'm trying to find a way to check it to verify it's working properly, just not sure how yet. But the idle actually dropped and the vehicle ran a little better after replacing it, so I think the new one is ok.
 
SanDiegoXJ said:
So I just failed smog with ungodly high NOx on a newly rebuilt engine. 1994 Jeep Cherokee Country 4.0 Auto.

Since the rebuild, the engine has suffered from a vibration at low torque, and has had what felt like a miss at WOT. However, it idles very smooth.

I've checked for vacuum leaks with starter fluid (I know, not the best method, but it usually works). It has new plugs, wires, cap, rotor, distributor, IAC, TPS, CTS, water pump, coolant hoses, radiator is 3 years old 3 core.

The overall consensus till this point has been air getting in the system somewhere, but I just cant find it.

Then I stuck a pressure guage on the fuel rail and fired the Jeep up. Took a look, 31psi...good news. Popped the throttle a couple of times and the pressure bounced between 31 and 38...about normal for what I know. Then I slowly applied the throttle and the fuel pressure decreased from 31-30 and stayed there (at this point I could feel the vibration kick in again). As I applied a little more pressure to the throttle, the fuel pressure dropped a hair more between 29.5 and 30 psi and the vibration intensified...from there, I could continue to apply pressure but you could still feel what I think is a miss. When I let off the throttle, pressure bounces just a bit and normalizes quickly at 31psi.

Is the decrease in pressure normal? Is it indicitive of a bad pump, clogged filter, or bad regulator?

Thanks for any input.
Click to expand...

All that fuel pressure data is normal to my knowledge, and is what I see on my Renix jeeps. Have you checked the flex plate for a loose bolt?????
 
Can you please post all the smog test data?

Also, is anything non-oem non-stock?
 
SanDiegoXJ said:
yeah, already did that too.

31 psi with, 39 psi without

The real question is should the fuel pressure in the rail ever be lower while under load than it is at idle.

basically, what is correct fuel pressure at say 1500-2000rpm. That's where I notice it most. It's also the same point that pressure drops from 31psi to 30psi, and it's most noticeable when warm.

EDIT: Has a brand new cat too, forgot to mention that. I know it's a lean running condition...so I figure what I need to do is find out what psi the fuel rail should have under higher rpm, get an ohm meter and check the resistance on each injector plug, (injectors are 2 year old remanned mustang injectors), and check the resistance on the o2 sensor to make sure I didn't just get a bad sensor.
Click to expand...

My 87 reads as low as 29 psi, no problem there.
 
SanDiegoXJ said:
yeah, it has a new 02 sensor in it. I'm trying to find a way to check it to verify it's working properly, just not sure how yet. But the idle actually dropped and the vehicle ran a little better after replacing it, so I think the new one is ok.
Click to expand...

O2 sensor testing:

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+sensor+test&page=7

Here is a very good, very recent thread solving your problem!!!!


http://www.naxja.org/forum/showthread.php?t=1077191&highlight=oxygen+sensor+test
 
unfortunately no, I can't post the smog data at this time. For some reason the lady at DMV made me turn it over to her in order to get my 60 day extension.

I can tell you that HC and CO were well below max and ave, while NOx was 1200 at 15mph and 1180 at 25mph.

As for the stock, non-stock, the engine is a Jeep 4.0 that is rebuilt 30 over with a new cam, borla header and cat-back, but other than that, it's all stock.

Usually a new cat will resolve the issue, raising the HC and CO but lowering the NOx and bring it all into spec again.

The only thing I noticed after the smog inspection was that the aux fan was unplugged so it wasn't kicking on over 215.

The Jeep doesn't run hot however.

And yeah, I saw that other post, but if I unplug my CTS, it throws a check engine light and I fail the visual inspection.

A buddy suggested I put a bb up in the fuel regulator vacuum hose causing it to run higher fuel pressure hoping to run it a bit richer and coold down the combustion chamber.

The one commonality I see between a lot of the other posts is aftermarket headers. But again, I've always been able to fix this with a new cat, so it strikes me that something must be way off somewhere else.

I'll get to that O2 sensor test today or tomorrow.
 
215? Just what do you consider hot, or normal?

Did it have a new cat already?

Try a real dirty, oil soaked air filter, to maybe counteract the headers? LOL.
 
Ecomike said:
215? Just what do you consider hot, or normal?

Did it have a new cat already?

Try a real dirty, oil soaked air filter, to maybe counteract the headers? LOL.
Click to expand...

Oh, maybe I'm wrong, but I thought the aux fan was supposed to kick on at 215....whatever it is, it rarely hits that point unless it's been running for hours. Unfortunately, the internal gauge is off, so I don't know exactly what temp it runs at either...just gauge it from where my old engine use to run.

Yes, it already has a new magnaflow CARB approved cat. And yes, I did run drive around for about 30 minutes to get the cat nice and hot.

And honestly, I've been thinking the same thing as far as the air filter. Right now I have K&N (oops, forgot to mention that)...may go back to a FRAM since it's probably a bit more restrictive.
 
I couldn't get my temp gauge to read correctly untill I got a new sensor from the dealership. All the parts house sensors read differently and al least 20 degrees off.

All sensors will now be replaced with Mopar ones when they need to be replaced. I was also told by the smog tech I went to that the Mopar O2 sensors have twice the response rate that Bosch ones do, so if you need a new one go get it from Mopar.
 
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I'm thinking EGR System with the high RPM Miss, EGR possibly leaning out the engine to much. How do the plugs look

But if u have a friend with one of those Barbecue Lazer Temp guns you can rule out Over Heating.

Turning on your electric fan, and having the smog guy put there big fan infront of your jeep could help with clynder temperature
 
no EGR on the '94 HO. I'm starting to think the problem may just be a very clean burning engine with hi flow air filter and hi flow exhaust. Gonna try a more restrictive air filter and some cooler plugs and see how that goes.
 
blubullett said:
I couldn't get my temp gauge to read correctly untill I got a new sensor from the dealership. All the parts house sensors read differently and al least 20 degrees off.

All sensors will now be replaced with Mopar ones when they need to be replaced. I was also told by the smog tech I went to that the Mopar O2 sensors have twice the response rate that Bosch ones do, so if you need a new one go get it from Mopar.
Click to expand...

Heh, wish it was that simple....the guy I bought it from tried to manually adjust the temp guage (it used to get stuck all the time he said...later I realized he had a faulty sensor...anywho, I have no idea what it's reading now, but at dead cold it sits well below the minimum mark on the guage =D)

And yeah, I found the same thing on my Wrangler, it really liked mopar sensors.
 
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