Help. Code 334 and low gas mileage. Bad EGR? Vaccum leak?

sj9ers

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May 19, 2007
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I have a 95 Mustang GT with 110k miles 5 speed, 2.73 gears, 295 rear tires, and only mods to the engine and exhaust are BBK cold air intake and off road pro chamber, and it has O2 sensors that are little less than a year old. Ever since I got the car my gas mileage has been about 170 or 175 miles to a tank is this normal? I don't beat on the car so I don't think it's because of that, but I do a lot of city driving.

Anyway a few weeks ago the car started acting up after I did a tune up with new cap, rotor, spark plugs and wires, and set the timing at 11 degrees. It started stumbling a little bit in lower RPMs and my gas mileage seemed to go down. To try and fix the problem I re gapped the plugs to .052, got a new TPS cleaned the MAF and the IAC, and even got a new distributor. While the car does run a little bit better the mileage seems to have dropped lower than my usual 175 a tank, and after a while driving the car seems to stumble a little bit in lower rpms.

I pulled codes the other day and got code 334 for the EGR voltage being too high at ignition on. So I tested the valve and applied vacuum to the valve while the motor idled and it stumbled and died so it was good. Then I replaced a bunch of vacuum lines under the hood and the one from the vac canister.

Before I go out and buy a new EGR valve sensor I was wondering how I could check the vacuum on the lines, specifically where to check vacuum at and what their readings should be?
 
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Don't know what the vacuum should be, so I can't help you there. If you do get an new EGR sensor, just know that you can buy the plastic EGR sensor separate from the EGR valve itself. Used to be you had to buy both together for over $100.

170-175 miles to a tank isn't too horrible for mostly city driving, but it depends on how many gallons you mean by "a tank". I tend to fill up at a quarter to an eighth of a tank; about 11-13 gallons. MPG is a better estimate; reset yuor odometer at each fill up, then divide the number on the odometer by the number of gallons you put in. I average anywhere from 16-20mpg depending on how many freeway miles I put on and how many times I spin the tires. :D
 
to check egr valve:

to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.

if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 rpms (remember snap the throttle don't hold it there).
did the vacuum gauge show about 2-5in vacuum?

if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500rpms.
should read about 2-5in vacuum

there should be 0 in vac at EGR valve with engine idling.
 
So I replaced my EGR sensor with one I had in my garage from a fox 5.0 yesterday so that should be good. I checked the valve and applied vacuum to it and the motor stumbled and stalled out so that's good. I also checked the vacuum line to the EGR and it checked out good and I got no vacuum at idle and 5 HG when I snapped it to 2500rpm. So right now my EGR system should be good.

However my mileage is still kinda low. I drove to school and back today and I'm already at half a tank with 80 miles. And I still don't know whats wrong. Here's the issues I've been having
-After driving for a while and warming up the idle seems to be a little rougher and in low rpms will stumble when pressing on the gas, but not all the time. in Higher rpms above 1500 the car runs absolutely fine.
-Sometimes the car is hard to start and it takes a couple of tries to start it

What has been replaced cleaned or checked
New Motorcraft copper plugs gapped at.052
New Ford Racing 9mm Wires
New MSD cap and Rotor
New distributor
Timing at 11 degrees
New TPS set at .98 volts
New Vacuum lines
New Battery
Alternator a little over a year old
Cleaned MAF, IAC,
O2 sensors a little over a year old
Fuel filter is less than a year old
Fuel pressure is good
Reset idle and computer a couple of times already
TFI module good
I'm at wits end at what to check or replace. The only thing else I can think of is to replace the coil since the rest of the ignition system pretty much has been replaced or checked out. Would a coil cause the problems I'm experiencing? It's an MSD Street Fire coil that the previous owner put on, and I don't know how old it is.
 
for the rough idle: check for vacuum leaks and check/clean the IAC.

fuel problem:
what was the fuel pressure...KOEO(should be 35-45)?? engine idling(should be 30-40)??

check that the AIR (smog pump) vacuum system is routed correctly.

for the intermittent hard start: when it won't start, turn the key ON for about 5 seconds then OFF for 10 seconds, repeat 2 or 3 more times and try to start the car.

for the coil: normally when a coil starts going bad the higher rpms suffer first (misfires), since yours are fine i would expect the coil is ok.

do you have somg inspections in your state?? do you know anybody with a scanner??
 
for the rough idle: check for vacuum leaks and check/clean the IAC.

fuel problem:
what was the fuel pressure...KOEO(should be 35-45)?? engine idling(should be 30-40)??

check that the AIR (smog pump) vacuum system is routed correctly.

for the intermittent hard start: when it won't start, turn the key ON for about 5 seconds then OFF for 10 seconds, repeat 2 or 3 more times and try to start the car.

for the coil: normally when a coil starts going bad the higher rpms suffer first (misfires), since yours are fine i would expect the coil is ok.

do you have somg inspections in your state?? do you know anybody with a scanner??

For the fuel pressure I didn't check it, but the mustang shop I did and they said it was fine and I also already cleaned the IAC about a month ago when I first trying to diagnose the problem. I live in CA so I do have smog inspections.

By scanner do you mean a code scanner? I do have one and I've pulled codes a few times already. I once got a 157 and 158 for MAF being too high and too low, but I suspect it was from me cleaning it. After I reset the computer I never got it again.

I noticed a few new symptoms. When I first start the car sometimes the car idle will shoot up to 1700rpms then hang for a little bit then fall down to 1000 for a bit, but then go back up to 1200-1500 then fall back down to 800rpm. This all happens within about 20 to 30 seconds. Also after I get to operating temp the idle gets a little rougher and I have a little surge in my idle.

I still have to check for vac leaks I couldn't because it's been raining, but could my IAC be the problem? My problem only seems to get worse after I drive around for a bit, and if I had a vac leak shouldn't the symptoms occur all the time even before the car warms up?
 
by scanner i mean something that can read the data that the ECM sees. it's more advanced than a code reader.

to see if the car is actually running rich you can ask a smog shop to check the emissions on your car (NOT a smog inspection). they may or may not dyno it, if they don't then ask for and idle and 2500rpm check. i would need to know the CO, CO2, O2 and HC. if they run it on a dyno then i'll need NOx also. if you live in OC i might be able to talk one of my friends into doing it.

the first 20-60 seconds that the engine runs is called OPEN loop. it runs a preset tune using TPS and MAF. after that time, and normally when warmed up it runs in CLOSED loop where it uses a different tune and also uses the O2 sensors and makes adjustments accordingly.

depending on where the vacuum leak is, it could make it run worse after warm up because of the above. if you have a misfire then the O2 from that cylinder isn't burned and will cause the ECM to think that the engine needs more fuel. also, anytime that cylinder is suppose to fire, it could cause a drop in rpm.