p30 stumble problems/ fuel pressure regulators

[lonfu]

Ok, so having a problem with up towing stumble when under load. I have 98 454 "j" 26wu Itasca on the last year that chevy produced it's P30 chassis. This stumble has been a real bummer, I checked the fuel pressure when the vehicle was new and it showed 52psi. I added an additional gas tank and it dropped due to the switch over valve to 50psi. shop manuel says 56 to 62psi. But I have driven it 120k with the 50psi with no problems. Checked recently and it has dropped to 48psi. Changed the Fuel Pressure Regulator to a stock napa unit, and got the reading of 46psi. Huh?????

Turns out the original unit is adjustable. They also make the fixed 50psi unit and also a 56psi unit for chevy trucks. Been trying to find one of the original units, so far no luck. Due to the length of the fuel lines and fuel switch over valve, the 56psi unit will show only 54psi, which is still below factory specs. So, turns out these adjustable units are what they used to adjust for the fuel line length and supposed to adjusted by the manufacturer. Duh.... needless to say, they were mostly not....

Yes, I changed out the fuel filter and fuel selector valve. Both pumps are showing the exact same pressure at the engine, so it is not the pumps. The only way to get the Fuel pressure to specs is to get the correct FPR that that is pre-set to 65psi to allow for fuel pressure drop or find and adjustable one. Any suggestions as to a source for the correct FPR?

[lonfu]

you Won't believe this, getting mis-fires again, then the upstream bosch sensors started reading low????? one has completely failed. Huh???? Not even 6 month old...... I replaced both upstream sensors with AC Delco's as I did the down stream units and all is well again, no-misfires, wow I'm really surprised at these thing failing. Auto zone stood behind their warranty and refunded for the bosch units. What a great company to do business with.

[lonfu]

Update to my update!!!!!

got an engine light again. Hooked up my teck2 code reader and guess what? I have another brand new bosch O2 sensor failing, bank 2 sensor 2. So, I ordered 2 new AC Delco units and replaced both downstream sensors. All back to normal. Returned the Bosh to Autozone and they refunded me my $$$$. DANG, for both sensors even if only one was bad!!!! Guess that is why I keep getting parts from them!

[allbrandauto]

fuel trim of -5 to-7 is not bad I would be thinking more of a vacume leak maybe from intake manifold gaskets if you could find somebody that had a smoke machine it would be easy to see

[lonfu]

Update: Checked my engine readings. still getting negative fuel trim values, -5 to -7. Coach seems to be running perfect. Idle smooth, temp in a good range, gas mileage varies between 8mpg over passes and 11mpg the flats of the desert.

I'm tempted to replace the idle mix valve to see if that is what is causing the negative values. Any one have any experience with this?

[lonfu]

P30 stumble fuel pressure update: Took a 1400 mile walkabout. Fernley Nv to Sacramento and back, over Donner pass. I got 8.25 mpg, fully loaded with toad. Fernley Nv to kingman az just below 11mpg, this is a pretty flat run. No problems at all. Lots of power, brakes worked perfectly, engine temp never going over 200 but outside ambient was in the 50's going over Donner pass.

Down town vegas was another matter through. Ambient temp went to 114, turned on dash AC, engine temp went to 220 mechanical. As the ambient temp dropped to 98 when we drove up the 95 and the engine temp dropped to 192. Did I mention it was stop and go 15 mph traffic?

I mentioned before, I had a still had a slight misfire on cylinders 1 and 2. I pulled the bosch plugs(11k miles) and replaced them with AC delco's iridium plugs. Only one missifire per 32 cycles on cylinder 8 when engine is cold and that goes away. Guess my old 454 vortec does not like german engineering!!!! Or bosch parts are crapola )-:

I replaced the up stream O2 sensors and the -fuel trim reduced from -7% to -4% (+-1%). I can't figure why I would be still getting a negative fuel trim unless it is due to the elevation that I'm at. 3500 feet. Replaced the vacuum sensor and the ECM temp sensor. I replaced these sensors because they were showing a weak voltage and a low engine temp on the tech2 code reader. Because of the new fuel injectors I now have a very smooth idle and improved gas mileage. After the problems with the bosch plug wires and plugs, I'm a little concerned that the bosch injectors will give me a problem down the road a bit.

The fuel trim still troubles me, I pulled the idle selinoid, cleaned it, tested. All is perfect so I figure out why the computer would reduce the fuel? Yes, I know what the books say: low fuel trim, poor air intake or 02 sensor problems, but my sensors are all new, exhaust system is sealed, and the 02 sensors are new and reading the correct voltage. The low -negative fuel trim is only at idle, not at 1500rpm's. I'm sure most of you remember that high fuel trim +percentages mean a vacuum leak or additional air is getting in past the MAS and MATS so you get fuel added to maintain the 14.7:1 air mix. Your ECM programming sets your fuel injector pulse width but then alters it slightly when one of the sensors tell it there is a problem with the mixture. Mine is negative, so that means it is reducing the fuel (pulse width) due to less air? Checked the air filter to see if it was clogged, it was clear.

So, I'm out of ideas for the - fuel trim, any suggestions?

Anyone have any suggestions as to why this is happening? Any practical experience with this elevation and -fuel trim?

Thanks

[sjholt]

At that mileage, my 86 gasser with a 454 had two exhaust valves receding, making it miss.
I replaced the heads and she ran good again.

[lonfu]

RCL wrote:
I had an '83 Southwind years ago which was prone to "stumbling" mine was caused by "cross firing" between cylinders. When the ignition wires aged, the spark could jump between a firing cylinder and one which was in the intake cycle, causing a sound like a "sneeze." New wires would usually fix it. I ended up buying NAPA lifetime wires because they would exchange them with no questions. I also found in a manual somewhere (don't recall where) that the routing of the ignition wires was important, keeping the wires between firing and intaking cylinders separated to minimize the cross firing.
One thing to realize is that the voltage required fireaplug goes up as the pressure increases, which is the case when the fireing cylinder is under full throttle. That makes the spark more likely to find another path. My "sneezing" always occurred under load condition.

Dick L

good point about the routing. I use wire loom to separate my wires as well as to keep them off the hot valve cover's. The loom seems to stop the cross fire problem. Of course I use the original wire stand off's (holders).

[RCL]

I had an '83 Southwind years ago which was prone to "stumbling" mine was caused by "cross firing" between cylinders. When the ignition wires aged, the spark could jump between a firing cylinder and one which was in the intake cycle, causing a sound like a "sneeze." New wires would usually fix it. I ended up buying NAPA lifetime wires because they would exchange them with no questions. I also found in a manual somewhere (don't recall where) that the routing of the ignition wires was important, keeping the wires between firing and intaking cylinders separated to minimize the cross firing.
One thing to realize is that the voltage required fireaplug goes up as the pressure increases, which is the case when the fireing cylinder is under full throttle. That makes the spark more likely to find another path. My "sneezing" always occurred under load condition.

Dick L

[lonfu]

yes, I was correct. I replaced both exhaust man gaskets. Bank 1 where the leak was so very bad, was a flypro gasket and it had completely failed. The center core of it disintegrated. The original GM gasket on bank 2 was still in medium condition after 124k miles, but it was leaking due to a burn through. GM gaskets are $30 each vs. $16 for the flypro pair. I'm using grade 8 studs with a stainless steel washer, lock washer and short brass nut on each stud. The lock washer by itself is to small, it catches on the edge of the hole in the exhaust manifold. 2nd brass nut to hold on the spark plug shield cover on the stud. Double tall brass nuts on the rest of the non-shield studs. Fired up today, cool over night, re-torque, then finish assembly.

The mis-fires on cyl. 4 and 6 are totally gone, but still seeing a very small misfire on cyl. 1 and 2. I'm thinking that perhaps the upstream 02 sensors are failing. They appear to be giving a strong voltage, but they are 17 years old with 125k miles. I've decided to replace them. Engine seems to idle smooth once it warms up, better than ever. But still thinking of those misfires.

Did I mention I checked the compression on all cylinders? all were at about 140 with all plugs out except 2 that were at 130. When I added oil they came up to 134, no surprise. I can't figure out how I got 148psi in 4 an 6 earlier. I did a leak down test on the low ones and found them at 20 to 24%. Norm new is about 10%. No cross cylinder leakage, So valves are probably weak, big surprise for 125k.

[lonfu]

My continuing stumble adventure....

So I bought a tech2 Chinese clone code reader direct from china. I'm having a little problem reading the Chinese but it does seem to fire up and make all kinds of interesting Chinese ideograms.

Got ya! Actually it works great and gives me access to all the engine data available including the O2 sensors and most importantly the misfire data. $260 Incl. ship. It actually includes a misfire graph. Yes, it is in English!!!!!

I found that cylinders 4 an 6 are misfiring with 1 and 8 occasionally missing too. Found that the bank 1 of the 02 sensors was reading real low, so I replaced both downstream sensors. The 02 reading came up some but the bank 1 was still very low. Cylinder pressure at 4 and 6, bank 2, both at 148psi, dry.

Then I hooked up my huge old shop vac to the exhaust pipe, engine cold. (the out flow) and found a huge exhaust leak on the left, bank 1, exhaust manifold and a small one on the right bank 2. Which explains the low 02 sensor readings, left bank 1 and the misfire on 4 and 6, right bank 2. (they are trying to lean the mix to compensate for left bank 1) I decided to replace the fuel injectors due to the moho mileage (124k) and the misfires on 1 and 8 as well as 4 and 6, guessed partially plugged up injectors. When I pulled the exhaust manifold on the left bank 1 turns out some of the new studs that I had replaced, while replacing the manifold, were to long, and prevent me from completely tightening down the manifold. Yes, correct torque. This was a surprise because I got them at my local napa store. Usually these guys are right spot on for everything but this time they screwed the pooch. I got on line and found the correct exhaust man studs were listed as not compatible with my 454 vortec. Hmmmm.... The felpro gasket that I installed had 2 layers of stainless with a fiber material between. Found that the original gaskets are 3 pieces of stainless, no fiber material but cost twice as much. Went with the originals. When I removed the right bank ex manifold I found a cracked manifold underneath facing the engine, so ordered a new Chinese part. $127 from Autozone with discount. While waiting on gaskets to finish the exhaust manifolds I had some routine basil cell skin cancer surgery. I now have a 2 1/2" incision on the left bank 1 from my eye to the top of my ear. Eye is swelled closed and wife has threatened to clobber me with her fry pan to give me a matching scar the right bank 2 if I work on the moho before it heals up. With my degrading eyesight at least I'll be able to see that big "ole" fry pan coming at me! Thank the Lord she likes the aluminum ones and not cast iron like my mother.......

Ahhhh!!! Are you are wondering about the fuel system and the effect of the new injectors on the fuel pressure? Ok, first of all I got a response from a Delphi engineer when I contacted him by email. Specs for this engine is 56 to 62 psi. Key on engine off. Turns out their units are set at 60psi. But with the engine running should be at 51psi due to the fact that it is a "vaccum" operated fuel pressure regulator. Before the new FI's it was running 46, after the FI's it jumped up to 49 when the engine is running and hot. I can live with this due to the length of the fuel line. I also contacted AC Delco, but they wouldn't give me the time of day or anything else for that matter. With new FI's the engine idles smoother. Since I'm still healing and in the middle of the new exhaust manifold, I will have to add more later.

Lastly, I did replace the TPS, MAS, MATS, EGR,PCV, MAP, Coil, Ign Module, both CC's, and the muffler. I believe the TPS was causing the start out stumble but is amplified by the bad 02 sensor reading due to the leaking exhaust manifold gaskets. AND.... Because the exhaust manifold gaskets failed, the spark plug wires were trashed on the plug, but they were still working correctly but the spark plug covers had turned to goo!

It was going to be such a simple little surgery to take out a little spot of cancer....

[MountainAir05]

I also think, electrical, but also check the fuel caps, vent system.

[outofplace]

If it revs fine, goes to upper RPMS when you gradually accelerate but breaks up under load i would change the coil on the distributor cap. I feel if it was fuel pressure related it just wouldnt rev very good because its not getting enough fuel but I dought it would stumble

[DSDP_Don]

That engine and chassis were notorious for burning plug wires and would have issues under load. Are you sure it's not getting fuel or is it missing. The plug wires would get overheated by the exhaust manifolds due to lack of air flow around the engine.

I would check the plug wires. If they are bad, many took dryer hose and routed some air from the grill across both manifolds.

[crasster]

I'm going to throw one to you because of the mileage. Try Marvel Mystery Oil in the gas. I'm curious if you have slight injector blockage (varnish) or your pump needs a bit of lubrication. MMO will fix those. Run it with manufacturer's suggested mix to your tank size.