TBI 350 - WOT Hesitation Under Load

[dhcomp]

Hey Guys,

Glad to have my burb back on the road, but unless i can fix this hesitation, its almost worthless to me in terms of towing.

Currently, when i've got the throttle burried, and the WOT RPM's stabilize for a few seconds, it starts to hesitate and bog down.

Slowly, it will drop 1-200 rpms, hold for a few seconds, then drop another 1-200rpms, continuing until i let off to about 3/4 throttle.

The main issue is while towing with this tired 350, i need EVERYTHING its got to make it up some grades. My boat isn't heavy, probably 4k loaded, but i need to replace this truck and/or motor if it can't get this job done.

Any hints? VERY hard to get it to duplicate with no load, will pull grades WOT unloaded without hesitation.

Mileage of the motor is unknown, but it makes good oil pressure. I will have a crate 350 dropped in it if needed, but am trying to not spent much $$$, as i am trying to buy a house, and was keeping my fingers crossed that the old beast could be patched together for another year.

Thanks!

 

[smier]

I'm doubtful it's the motor, sounds more like fuel delivery, ignition, or computer related. I'm trying to remember, but I think at WOT the ECM is programmed to behave differently, but it's been a few years since I've played around with this stuff. I'd post over in the injection section if you haven't already. Does your check engine light work? Have you tried pulling any stored codes yet? Might be a good time to change the fuel filter and check your fuel pressure first... You could always datalog and see what's happening under a load at WOT.

 

[dhcomp]

Thanks for your suggestion. I'll post in the injection section...i keep forgetting they deal with stock stuff over there too.

Truck has New fuel pump, full CFM tech TBI upgrades, pressure set at around 13psi currently. Recent stock computer swap, filter replaced with pump in the last year.

EGR solenoid vac line is plugged at the TBI, as it was causing some funky surging at low rpm/speeds. Check engine light works, but EGR was the only code i was getting, with intermittent Check Engine Light. I'll pull codes again and see what i can find.

 

[4xcrazy]

Shouldn't need WOT to pull that weight...

Wrote something else, but won't let me delete on my phone.

Brainfarted and didn't read your entire post about the fuel pressure. That's always the first thing I check when something weird starts happening with these F.I. engines.

 

[dhcomp]

Shouldn't need WOT to pull that weight...

Wrote something else, but won't let me delete on my phone.

Brainfarted and didn't read your entire post about the fuel pressure. That's always the first thing I check when something weird starts happening with these F.I. engines.

I would like to say i don't need WOT to pull that weight, but other than flat highway cruising, the pedal stays pretty burried.

ESPECIALLY up a grade. Last summer, with the 3.73's, at WOT my speed dropped to 35mph for a good section going up to tahoe.



Wish i could see my fuel pressure while driving, but am surprised that it would drop just a WOT.

This being said, i had this issue before the CFM stuff. It has also worsened over the last year or 2, where now it keeps me from staying in the throttle.

 

[imiceman44]

I would like to say i don't need WOT to pull that weight, but other than flat highway cruising, the pedal stays pretty burried.

ESPECIALLY up a grade. Last summer, with the 3.73's, at WOT my speed dropped to 35mph for a good section going up to tahoe.



Wish i could see my fuel pressure while driving, but am surprised that it would drop just a WOT.

This being said, i had this issue before the CFM stuff. It has also worsened over the last year or 2, where now it keeps me from staying in the throttle.

Well I used to tow my trailer behind my truck up steep hills like pacheco pass and up I80 and even though I was geared low enough (4.10 and 31" tires) I still need to be almost at WOT, it felt the same wether I burried the pedal or had it a bit of the floor, I kept prscticing untill I got exactly the sweet spot, and it worked fine after that.
Beofre that it would hesitate after a few seconds of WOT because the fuel pump wouldn't have the flow to keep up, and it also would overheat quick.
I don't know if there are options out there for electric pumps with bigger flow, not higher pressure, but that is what you need for extended periods of WOT.

 

[smier]

Why do you have the egr solenoid line plugged? If you still have the egr valve installed, the solenoid no longer controls it. I'm wondering if your egr issues don't have something to do with it? I know when I plugged my egr solenoid line on mine and tried to drive my truck, it had no power and would actually ping under load... If I remember correctly, doesn't the egr system actually help prevent detonation at highway speeds under load? Is it possible that the ECM is retarding the timing to prevent knocking? I'm really just speculating here... Kinda thinking out loud.

 

[dhcomp]

The thing is, 2 yrs ago, i could pull those grades, foot burried to the floor, for 20+ miles w/o hesitation.

Now, it hesitates.

 

[imiceman44]

The thing is, 2 yrs ago, i could pull those grades, foot burried to the floor, for 20+ miles w/o hesitation.

Now, it hesitates.

old pump or new?

 

[dhcomp]

old pump or new?

The old one

I replaced it to deal with an idle stumbling issue (which ended up getting fixed with a new delco computer).

The replacement was a delco, and i replaced the little hose section and filter at the same time.

 

[Fordum]

It really sounds like fuel. Although I would expect it to set a lean condition code if it was. The only way you will know for sure, is to either shotgun it and replace the pump, filter, and maybe rebuild the regulator, or figure out a way to monitor the pressure while its bogging.

Oops, having said that, it might also be an exhaust backpressure problem.

smier is right, EGR tends to reduce or prevent knocking, and if the knock sensor is hearing a knock, its gonna do stuff to stop it, which will reduce power.

 

[dhcomp]

It really sounds like fuel. Although I would expect it to set a lean condition code if it was. The only way you will know for sure, is to either shotgun it and replace the pump, filter, and maybe rebuild the regulator, or figure out a way to monitor the pressure while its bogging.

Oops, having said that, it might also be an exhaust backpressure problem.

smier is right, EGR tends to reduce or prevent knocking, and if the knock sensor is hearing a knock, its gonna do stuff to stop it, which will reduce power.

Pump is new, filter is new, and fuel pressure regulator is adjustable CFM tech now, so neither of those should be the culprit. I could turn up the fuel pressure though.

Hm, i wonder if it could be related to the disconnected EGR.


What type of backpressure problem are you thinking? I'm running shorty flow tech headers from a 92+ TBI.

 

[Fordum]

What type of backpressure problem are you thinking? I'm running shorty flow tech headers from a 92+ TBI.

I was thinking of a plugged up cat. Friend of mine used to work in a vet's office and he said those were the worst..............

Any chance of getting a scanner type machine on it so you can watch the timing or knock sensor?

 

[dhcomp]

What type of scanner do you need for OBDI?

My cat was replaced while i've owned it, but i had a massive exhaust leak for a while, so it may not be in great shape.

Probably close to 30k ago.

 

[dhcomp]

Who's good at reading spray patterns?!


Had a bit of random liquid splattering before the weekend, barely noticeable. Then, after the weekend.....

Had my truck parked 2 ft from the garage door this morn, and it idled cold for about 2 min. Holy crap. What is coming out of this?!? Coolant and oil seem to not be dropping.


Read codes, have a 13 ( Left bank O2 sensor circuit open or no activity), and a Code 44 ( Oxygen sensor error (lean condition indicated)).

So, ordered a heated O2 sensor to retro fit.

No EGR code....

So, besides the o2 sensor.......whats up with this thing?

Why is it spewing when started up cold? What is different at cold start on what its reading, vs WOT?

 

[Fordum]

You haven't sucked any vampires up into the intake lately have you?.
Without being there to feel and smell it, I'm gonna go with water and carbon.

Water, because all IC gasoline engines make moisture, and spray water until the exhaust get hot enough to keep it in vapor form all the way, and carbon because it may be running rich and have the exhaust system sooted up.

 

[imiceman44]

You haven't sucked any vampires up into the intake lately have you?.
Without being there to feel and smell it, I'm gonna go with water and carbon.

Water, because all IC gasoline engines make moisture, and spray water until the exhaust get hot enough to keep it in vapor form all the way, and carbon because it may be running rich and have the exhaust system sooted up.

 

[dhcomp]

Well, thats good news! Think my cat is messed up now?

Think its worth fighting with my EGR system for the time being, or should i leave it plugged? WIll it hurt the performance?

THanks guys!

 

[imiceman44]

Well, thats good news! Think my cat is messed up now?

Think its worth fighting with my EGR system for the time being, or should i leave it plugged? WIll it hurt the performance?

THanks guys!

Well here is the deal, when you run rich, you burn the CAT and does get plugged, so the possibility is there, and it could explain the degraded performance.
It could have happened when you were experimenting with your new pump and adjustable pressure regulator.

 

[dhcomp]

Well here is the deal, when you run rich, you burn the CAT and does get plugged, so the possibility is there, and it could explain the degraded performance.
It could have happened when you were experimenting with your new pump and adjustable pressure regulator.

I'm guessing the most recent rich condition was due to the o2 sensor though. I'll put a new 02 in it, and see what happens from there i guess.

Really, For $120 or whatever a cat costs now a days installed, not a huge deal. More of a hassle to take it in than anything else.