Engine/Transmission (1998.5 - 2002) 85mph engine pulse

[JPLB22]

This is something that just recently started ( the most recent long trip ). I usually cruise empty between 75 and 85 mph.

On my last trip I noticed the engine sounded different at 85 than it usually does. At 80 mph the engine sounds perfecty normal ( 2125 rpm ). But when I accelerated to 85 mph ( around 2250 rpm ) and cruised there for a while I noticed the engine had a pulsating rythm that it did not have at 80 mph. There is no change in speed or rpm when it pulses. It still has plenty of pedal left, I can easily accelerate to 95 mph. The engine rythm is very noticable and kind of annoying. What might have changed recently to cause this?? I have no BOMBs except 4" exhaust, which was on before it statred doing this.



Thank You for any replies.

 

[slavallee]

I hate to be a storm cloud...

but it occurs to me that it could be the LP preparing to shuffle off this mortal coil. The increased draw on fuel at that RPM/load could be just a scosh above what the LP can draw. The pulse you feel could be the injection pump working harder to pump when the LP starts cutting out. I mean, I HOPE I'm wrong about this, but it wouldn't hurt to check your FP.

 

[JPLB22]

I am trying to get a FP gauge installed. I am currently going through the worst money crisis of my life right now. But anyway, I assume you mean that I need to check the fuel pressure at 85 mph. I know that it would be cheap and easy to use a hand held pressure gauge to check FP at idle, but I assume this may not show any problems. Since at idle the engine is not consuming near the amount of fuel as it is at 85. I too hope that it is NOT my LP. I still have 80,000 miles worth of warranty. But trying to get the dealers around here to do warranty work is WORSE than pulling teeth ( no anesthesia when working with dealers ). I'm sure that unless I have them do a fuel filter change and it doesn't prime, I will have to wait until the LP is completely dead and so is the VP44.

 

[slavallee]

You can try what others have done, tell the dealer that you notice a "loss of engine power at WOT" and/or a hard starting condition. I guess the most common place for them to start is by checking the fuel pressure. If you have low FP at 85, it is most likely off at least a little through the rest of the RPM range. They may tell you that you are still within spec though. KEEP AFTER THEM!



lots of luck

-Scott

 

[JPLB22]

Thank You Scott!

 

[JPLB22]

I do have another question.



Will the LP starting to go out coincide with an increase in fuel mileage??



My fuel mileage on my last five tanks has gone like this, 16. 5 mpg / 17. 3 / 18. 3 / 18. 8 / 17. 9 mpg. I had been stuck at 16. 5 mpg ever sinse I it drove off the lot. And then at 18,500 miles I start to notice this rythm at 85 mph and my mileage starts climbing.

 

[slavallee]

Not having gone through this dilema myself I can't say for sure. But, it certainly falls within the realm of possibility. Less fuel draw= less fuel burned= better MPG. Unfortunately it also means a straining VP44. Let me know how you make out at the dealer. If they give you the run around for too long, you may want to bite the bullet and replace the LP before any damage to the VP44 is done if you're confident this is the problem.



-Scott

 

[Turbo Tim 1]

Usually a bad lift pump will mean less fuel mileage if it is dead. If you get a hose and a gauge like the Briar Hopper set-up you can put the hose under a windshield wiper and test pressure on the highway. I made my own for like $25 with an A/C hose and a gauge from a fisher valve.

 

[slavallee]

Less mileage if it is dead because the non-functioning pump (impeller type?) will actually restrict flow. Perhaps one that is still working but at low output may not cause a restriction until the VP tries to pump more that what the LP can feed it (i. e. 85 mph on the freeway). I'm not claiming that I'm right here but this is my way of thinking



-Scott

 

[greyghost]

Friday the 13th (day of the bad lift pump)

Just posted on a different thread about lift pumps



When mine %$#@ the bed it went something like:



Would be hard to start (hot or cold)



Couldn't get it to smoke (mash the pedal and just get a small trace)



Noticed when I cycled the key from off to on but not to start. The Lift Pump will turn on for a few seconds. Well the pump would stay on longer and longer and ... (noticed this one day while next to my friends 01. His pump was on for about 3 seconds and my mine 20 seconds. )



High Speed stumble, it would idle fine and if you didn't jump on it to hard or get over 2000 rpms you wouldn't feel a thing.



MPG did change a little towards the end by maybe 1 mpg. But nothing that made the red flag go up. After the replacement the mileage was about the same.



I'm on my 3rd pump which I moved and did some other things to, lots guys have posted about this one and the solutions they came up with.





Garrett

 

[JPLB22]

turbo tim1...



That is a dang good idea ( FP gauge under wiper ) I would never have thought of that. Thank You!!



My truck starts just as well as it did on day one ( one crank and it fires right up hot or cold )

There is no smoke on mine either if I romp on it, but I am also bone stock except for 4" exhaust.

My LP runs for 3 - 5 sec. when key is turned on but not started.

There is no stumble at any RPM. Just the strange rythm at 85 mph.



I just got back to work from a short trip. While out I locked it out of O. D. and went up to 2250 rpm's which is about 53 mph in 3rd. I didn't hear the same rythm change as I do at the same rpm's in O. D.



So I wonder how much different is the load or the engines demand for fuel when in or out of O. D. and doing identical rpm's when unloaded on flat ground. Would wind drag alone be enough of a difference to cause a funny rythm do to LP going south.

 

[greyghost]

Best way is by the gage. I'm not sure how you 01 and 02 hook up your fuel pressure gages. (I'm sure some one else will chime in too)



But IMHO you should see 7 psi to 15 psi. 12 to 15 psi at idle and no lower than 7 psi at anytime. The lift pumps die slow and take the VP44 with them.



Garrett



PS If you haven't already, changing the fuel filter might be in order.

 

[SS]

Are you setting the cruise control right at 85? I believe that may be the max allowable speed, so it *could* behave strangely if you are at or near the limit...



Just a simple thought (and cheaper than a new pump )

 

[JPLB22]

I change my fuel filter every 7500 faithfully. It has been in the last 7500 that it has started the rythm thing. I'm sure that if I don't gauge it before the next filter change than I should notice if it is acting funny then.



I don't use the cruise ever. My manual said to wait for engine break-in before using the cruise for extended periods. I'm figuring between 20,000 and 25,000 is where I will start using the cruise. BTW I have 19,700 right now.



Seriously thank you for the replies gentlemen. This is a great bunch of guys on this site.

 

[2-ND TIME]

engine stumble

Please read my note in 911 but could be that the engine is defueling and or not reading the MAP pressure correctly could be starving for fuel also check the the lift pump and make sure that they enter all codes on the work sheet and keep along paper trail for the future if needed. Hope that it is nothing but keep on it . Only buy fuel at good fuel stops this help to limit the new fuel filters needed.