Driveability Issue
Thread Starter
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Posting Guru
Joined: Dec 2002
Posts: 1,297
Likes: 3
From: Billings MT
So I went and got a fuel pressure gauge, at idle 32 psi, pulled vacuum line off and 38 psi.
Put vacuum line back on and rev it up with the gauge attached and it dropped to about 28 psi. Don’t know rpm, I was manually operating the butterfly valve.
This seems odd. As the RPM increase, the vacuum should decrease thus calling for more fuel but the psi dropped(contradictory to the idle test).
I’m tending to think this is the sign of a bad fuel pump, at low volume the psi is ok, but as volume demand increases it can’t maintain adequate psi. The fuel filter was just replaced with a WIX filter about two weeks ago so that shouldn’t cause a restriction.
Thanks in advance.
Does that seem logical to everyone else?
Put vacuum line back on and rev it up with the gauge attached and it dropped to about 28 psi. Don’t know rpm, I was manually operating the butterfly valve.
This seems odd. As the RPM increase, the vacuum should decrease thus calling for more fuel but the psi dropped(contradictory to the idle test).
I’m tending to think this is the sign of a bad fuel pump, at low volume the psi is ok, but as volume demand increases it can’t maintain adequate psi. The fuel filter was just replaced with a WIX filter about two weeks ago so that shouldn’t cause a restriction.
Thanks in advance.
Does that seem logical to everyone else?
Fleet Mechanic
Joined: Mar 2013
Posts: 1,576
Likes: 41
From: Wyoming
^^^^^^
Agree
Probably fuel pump.
Note the 6 psi difference you got on pressure readings is = to about 12.2 in hg.
If you are below 1000 ft elevation than your engine vacuum at idle is low, fuel pump is weak or fpr is off a little. I like to see 17 to 18 in hg difference or 8 to 9 psi difference.
Agree
Probably fuel pump.
Note the 6 psi difference you got on pressure readings is = to about 12.2 in hg.
If you are below 1000 ft elevation than your engine vacuum at idle is low, fuel pump is weak or fpr is off a little. I like to see 17 to 18 in hg difference or 8 to 9 psi difference.
Logistics Pro
Joined: Nov 2004
Posts: 4,795
Likes: 47
I didn’t see it mentioned, but check to make sure your air intake/filter is clear.
I mention this because you say it started after it was parked for a week
I just finished dealing with a chipmunk getting into my air box and chewing up, in the end, three air filters. Engine ended up going into limp mode (with check engine light). All fixed now. Filter snorkel screen, new air filter, clean throttle body.
I mention this because you say it started after it was parked for a week
I just finished dealing with a chipmunk getting into my air box and chewing up, in the end, three air filters. Engine ended up going into limp mode (with check engine light). All fixed now. Filter snorkel screen, new air filter, clean throttle body.
Thread Starter
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Posting Guru
Joined: Dec 2002
Posts: 1,297
Likes: 3
From: Billings MT
^^^^^^
Agree
Probably fuel pump.
Note the 6 psi difference you got on pressure readings is = to about 12.2 in hg.
If you are below 1000 ft elevation than your engine vacuum at idle is low, fuel pump is weak or fpr is off a little. I like to see 17 to 18 in hg difference or 8 to 9 psi difference.
Agree
Probably fuel pump.
Note the 6 psi difference you got on pressure readings is = to about 12.2 in hg.
If you are below 1000 ft elevation than your engine vacuum at idle is low, fuel pump is weak or fpr is off a little. I like to see 17 to 18 in hg difference or 8 to 9 psi difference.
Tank is almost full, will have to hope it holds out long enough to get it where I can drop the tank and that won’t happen with 200+ pounds of fuel and I don’t have that many gas cans to fill.
Air cleaner is clear, took tube apart, took MAF out and sprayed with MAF cleaner earlier in process.
Thanks
Fleet Mechanic
Joined: Mar 2013
Posts: 1,576
Likes: 41
From: Wyoming
I’m at 3100 feet.
Tank is almost full, will have to hope it holds out long enough to get it where I can drop the tank and that won’t happen with 200+ pounds of fuel and I don’t have that many gas cans to fill.
Air cleaner is clear, took tube apart, took MAF out and sprayed with MAF cleaner earlier in process.
Thanks
Tank is almost full, will have to hope it holds out long enough to get it where I can drop the tank and that won’t happen with 200+ pounds of fuel and I don’t have that many gas cans to fill.
Air cleaner is clear, took tube apart, took MAF out and sprayed with MAF cleaner earlier in process.
Thanks
Thread Starter
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Posting Guru
Joined: Dec 2002
Posts: 1,297
Likes: 3
From: Billings MT
Well, so I think we’re narrowing down the issue.
FP idle, 32, turned off motor and after 15 minutes was 39. Didn’t seem to drop from there so I don’t believe it’s the fuel pump now. I will do the power brake test once a neighbor gets home. Still could be.
Since I live in an emission free state for now, I think I’m going to remove the cat as a test. That does make sense as it gets warmer something is causing a restriction somewhere and at high RPM there is enough flow to overcome, but at low RPM and low flow it builds up excessive back pressure which could be why the O2 sensor was trying to lean out mixture because exhaust was stagnant in pipe. Also explains why the exhaust gas is so hot. The previous owner had a 165 thermostat in it which could have fouled the cat and no idea how long it was run that way.
FP idle, 32, turned off motor and after 15 minutes was 39. Didn’t seem to drop from there so I don’t believe it’s the fuel pump now. I will do the power brake test once a neighbor gets home. Still could be.
Since I live in an emission free state for now, I think I’m going to remove the cat as a test. That does make sense as it gets warmer something is causing a restriction somewhere and at high RPM there is enough flow to overcome, but at low RPM and low flow it builds up excessive back pressure which could be why the O2 sensor was trying to lean out mixture because exhaust was stagnant in pipe. Also explains why the exhaust gas is so hot. The previous owner had a 165 thermostat in it which could have fouled the cat and no idea how long it was run that way.
Thread Starter
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Posting Guru
Joined: Dec 2002
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From: Billings MT
Ok, cat is out, straight pipe in. Seems much much better. Same routes, doesn’t have the hiccup and stutter, even at just partial throttle and much more responsive. Exhaust is quite a bit cooler, I can actually hold my hand over it for 30-45 seconds now.
I need to hook back up to scanner, it’s still got a burp at idle. May be due to removing the cat, but all the back pressure should be created in the choke down of factory y-pipe. So I think something is still a miss.
Thanks, definitely would not have thought about the cat without your help.
I need to hook back up to scanner, it’s still got a burp at idle. May be due to removing the cat, but all the back pressure should be created in the choke down of factory y-pipe. So I think something is still a miss.
Thanks, definitely would not have thought about the cat without your help.
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