For a while now, I have been trying to figure out why my '00 Expy knocks so bad. After going through a lot of other stuff, I decided to check the fuel pressure. According to Haynes, the pressure at idle should be 30 - 40 psi; mine is right at 30. Also according to Haynes, while under load (or with the vaccuum hose pulled off the regulator), I should see 40 - 50 psi; mine is right at 40. Let me know what your pressure is, if you know it please!

Should I consider these numbers good since I am at the low end of "correct"? Not only does the engine ping under load, it has started to stumble while accelerating - I have to nurse it slowly to highway speeds. During this stumbling, I watched the fuel pressure sit steady at 40 psi.

I have replace the fuel filter, EGR, IAC, plugs, cleaned the MAF and air temp sensor, and put seafoam in the gastank. I have even had the dealer do their "decarb" treatment. All sensors are within range and the engine light has never even flashed on.

Please help!!

 

That is correct. You should see around 30psi with the vacuum line connected and 40psi with it disconnected.

What kind of fuel are you using?

 

Thanks! I'm not sure if that's good news or bad...

I normally use 87 but when I tow the trailer (less than 3000 lbs. total), I mix with 93. The octane does not seem to make much of a difference as far as I can tell. I have had the truck since it was new and have never really done anything different - the pinging started to get worse at about 80 - 90 K miles and it now has 108K.

 

Lets start from the beginning here and go over everything piece by piece.

When does the truck ping? Is it affected by heat, cold, weather, etc? What have you checked or replaced and in what order?

 

Could it be an air leak downstream of the MAF?

 

MisterCMK et al,

I've had the truck since new and have always put 87 octane in the tank. Right about 85,000 miles I put bigger tires on the truck. These are BFG A/T 285/70/17. I did this because I wanted a D-rated, all-terrain tire and there were none available in the stock size (there are some now). So I did the math and these tires are 3% bigger than stock - big deal, right? Right about this same time, the pinging DID seem worse!

After I started asking around, I found there are lots of people with the same size tire that are NOT having the same symptoms. I assume that the bigger tires are showing a problem that was there anyway so I decided to look for something else (besides I just spent $1,000 on the tires and they are not going back!)

Around 100k miles, I decided to replace a bunch of stuff that might be close to end-of-life or MIGHT be related to this issue and (thanks to this website) I picked the most likely suspects. That's when I replaced the spark plugs (all looked to be in great shape) EGR, IAC, fuel filter, checked all the vaccuum lines, cleaned the MAF and ATS, and checked the vaccuum lines again. After all this money and effort, the truck ran EXACTLY the same (pinging included).

I also had the tranny flushed, replaced front and back diff fluid, and Xfer case fluid.

Between 85 and 95k miles, I had to be under heavy load like going up hill pulling a trailer (3000 lbs.) for the pinging to get real bad. After 95k miles, it seems to be getting worse. With a trailer, a hill, and an 87/93 octane mix in the tank, the pinging is enough to make me kringe - it's bad. This is the first summer it has had to shift into 2nd gear to make it up these hills (downshifts twice) so I know there is a loss of power (I only pull the trailer in the summer). That's when I had the dealer do their "intake cleaning and decarbinization" treatment - no change.

Also, about this time, I asked the dealer to put it on their machine to confirm all sensors were "within spec." (whatever that means). They tell me that my pinging is likely caused by the oversized tires - I just don't buy this!

Now at 108k miles, it will ping slightly on a flat strech of road with no trailer, WHEN WARM. Cold, it is much better but it does not have to be HOT either. After about 10 minutes of driving, it will start pinging when I accelerate. For the past 3 weeks or so it will occationally stutter and stumble while accelerating. In the past, I've had clogged fuel filters on other cars and, to me, it feels like a clogged fuel filter. That's when I put the fuel pressure gage on (while driving). While pinging and stubbling, the pressure will sit steady at 40psi.

What I know:

Starts-up great, everytime.

Idles fine 90% of the time. Sometimes it feels like it is going to die at a light. It can idle great one day and rough the next. This problem does not seem related to temperature as far as I can tell. I can not find ANY reason for when it decides to idle rough.

Can ping when warm or cold but is more likely when warm. Pings worse under load.

Will run rough under acceleration SOMETIMES. I can go 50 miles without a problem then feel the stumble/stutter again while getting on the highway the next day. The rough acceleration always accompanies the rough idle.

I flushed the radiator at about 60,000 mi. and the temp gauge still sits where it always has (I think).

Normal paper Air filter is new.

Sorry about the long post but I can't think of anything else to tell. I am open to any and all suggestions. I am thinking of removing and cleaning the injectors next. Thoughts on this?

Thanks all!

 

Have you replaced the engine coolant temperature sensor? The vehicle relies heavily on this sensor in combination with others to determine spark, fuel and other parameters for engine operation. It might cause these goofy problems. They are fairly cheap as well.

 

Sounds like an easy thing to try; and easier than cleaning the injectors... Thanks for the suggestion, I'll let you know...

 

I'm suspicous of the knock sensor being bad. My '98 F150 acted the same way,and the knock sensor is bad. I simply disabled all knock sensor advance and adjusted the borderline knock table in the EEC programming to compensate. Problem is now gone.
You'll have to have a good quality scanner(or a programmer) with datalogging capability to determine if the knock sensor is bad. You'll need to look for a data PID called knock sensor retard. Watch the value of this PID while driving and in an audibly pinging situation. if the value of knock sensor retard does not change when the pinging starts,then the knock sensor is bad. Where are you located? Maybe somebody nearby has an Xcal2,and you guys can get togther and datalog it.
JL

 

That's a good idea. I'll try another post to see if anyone in my area (Cleveland, Ohio) is online and willing to share.

 

"I had the dealer do their "intake cleaning and decarbinization" treatment - no change."

Would this treatment also help a blocked catalytic converter?

 

Nope. A clogged/blocked cat needs to be replaced. I am dealing with the same thing right now and just ordered a Predator for the truck and will be installing my own "custom" cats.

 

heterodox1, any news on the problem?

 

No, I was driving with my wife in the car yesterday and she asked me "what's that rattling sound?" I just said "boy, would I like to know". I'm getting ready to go camping in the hills of West Virginia (with the trailer) next weekend and I'm a little scared to see how it will behave.

I posted in the tuner forum asking to borrow a datalogger from someone in the Cleveland area. Aren't these around $400? Tempting...

 

Did you change out the ECT?