TBI breaking up in at higher RPM, stumped

[ryan_thompson87]

Alright I don't normally post my issues, but I'm beginning to be stumped by this one. Engine is a 355 Vortec TBI with a GMPP intake, Melling cam, Ultimate TBI mods, including a vacuum reference fuel pressure regulator, EP 241 delco pump, MSD wires and yadda yadda longtubes and usual ****. Before anyone mentions the tune, been running the stock tune for 5 years now without a single problem, so I know that's not it.


Bout a month ago my truck developed a stumble in the upper RPMs (2500 and up) under heavy load, IE 3/4 throttle. If you drive like a sane person (which I usually don't) you don't notice it. At first it was bad enough to pop out the pipes, now it's changed to more of lighter chug and you can see the tach drop and it struggle. Does it much worse in 2nd and 3rd gear, the more load the worse it is, like in mud.

So far I have checked.changed:

-Fuel pressure set to 13 psi at idle and increases to 23ish at WOT (Haven't verified under load, gauge won't reach) Tried raising the pressure to see, did nothing.
- When the pump is deadheaded pressure jumped to 60psi
-Changed fuel filter and bypassed tank crossover (was beginning to close off and make noise like a bad pump)
-Timing still set to 15* Base.
-Needs a cap and rotor(mild corrosion) but is not the issue. No carbon tracking in the cap.
-Pickup coil is 800 ohms, wires not broken.
-Changed ignition module with a known good one.

I've got a fuel pump on order, because I secretly suspect it's dropping pressure under load, or the flow just isn't there. (Again the hose on my gauge isn't long enough to put on the cowl so I can see while driving) I put the EP241 pump in about two years ago with a new strainer, so I don't suspect the strainer to be plugged. I plan to check the plug wires and plugs (Wires are older 8MM MSDs and the plugs are about 2 month old Delcos) but other than that I'm running out of ideas. Engine is about two year old rebuild, so intake gaskets and everything are fairly new, truck doesn't get alot of miles put on it, just hard ones. Anybody have any other ideas on what might be causing it?

[GreaseDog]

Check your distributor gear. Mine was breaking up real bad about a year ago, and the teeth on the dizzy gear were severely knife edged. Changed the dizzy for another one I had here, and the problem went away. Ran ok, no real problems, then a few weeks ago I put a meloni zed gear in it, and it again made a difference. I was wearing into the 2nd gear.

[ryan_thompson87]

Check your distributor gear. Mine was breaking up real bad about a year ago, and the teeth on the dizzy gear were severely knife edged. Changed the dizzy for another one I had here, and the problem went away. Ran ok, no real problems, then a few weeks ago I put a meloni zed gear in it, and it again made a difference. I was wearing into the 2nd gear.

Will do, the distributor has to come out anyway, one of the damn screws broke when I was pulling the cap. :lol:

As I was going across town to get my scanner from work, I tossed a MAP sensor I had on, No change. Put a timing light on it, moved my timing back to six degrees. No change. Verified it was advancing with RPM.

Scanner showed nothing long and short term fuel trim were 128, 02s were switching normally, TPS,MAP, and ECT were all reading correctly and linear. Still have to pull the plugs and check them and the wires, didn't have time tonight. After that if I don't find anything I'll put this pump in and see. Also have another distributor I might steal off another truck to try, another suggestion I got was a cracked reluctor wheel in the dizzy.

[ryan_thompson87]

Had the plugs out they're all ok, wires all good, compression was good, +/-140 psi.

Pulled the dizzy to check the pickup coil for cracks in the reluctor and the gear, both were fine. Leaning back towards the pump. Have a spare coil I might toss on to just for ****s and giggles.

[Doober]

I was going to mention the coil... mine has a miss around 1800-2100 while cruising at that RPM (aka 75mph+, 3.08s and 28" tires make for low rpms...) or under mild/mediocre acceleration. I have an MSD coil with probably up around 100k miles on it now, still need to check resistances on it.

[ryan_thompson87]

Someone else mentioned it too. Cheap as they are I'm just gonna replace the pickup coil while the dizzy out.

[ryan_thompson87]

Weeeeell threw a distributor in and runs better than it ever has, f'ing eh.

[oldred95]

Mine did this for years. I tried everything from fuel to ignition and nothing made any real improvement. I ended up going to a big cap HEI for like a early 90s F body and it still didn't help. Tried a spare PCM and so on. Finally one day I was doing repeated WOT runs trying to get an ear on it and figure out what it might be and it started knocking. I kept on doing it to make sure it was really knocking and it was. Now the engine and tranny are out of it and the project is on the back burner waiting for funds for a 400 small block single turbo build. Will be switching it to OBD2 0411 PCM tuning it with efi live. The bastard had better run when its all done.

[ryan_thompson87]

I actually read through like 12 pages of a thread of yours on another forum with something similar :lol:

[oldred95]

Yea that was on thirdgen. Its like when you've got a transmission doing something completely stupid that you've never seen before. The fluid is perfect, no trash in the pan, no DTCs, its all good. You tear it down, every single thing looks like new. Replace a couple seals and gaskets and what not, put it back together and it does the exact same damn thing. The only way to fix it is to get a completely different one and start over. Thats what I will eventually be doing, getting something completely different and starting over.

[ryan_thompson87]

We have a 48RE at work in out of a Cummins right now like that :lol: Good example. Trucks still running perfect, btw.

[Doober]

I had a guy post in my thread on MalibuRacing.com about my issue that it was the ignition module. My distributor is from ebay and it started right up the first time. I think the heat sink compund may be drying up, etc. so I plan on replacing it with some good compound and going from there.

[Fast305]

I had a guy post in my thread on MalibuRacing.com about my issue that it was the ignition module. My distributor is from ebay and it started right up the first time. I think the heat sink compund may be drying up, etc. so I plan on replacing it with some good compound and going from there.

With a MSD 6A box, I have never had an igniton related break up. Its ALWAYS been FUEL. TBI injectors MUST stay below 80% duty cycle, otherwise it will do all kinds of crazy breaking up at higher RPMs. To run 6,000 rpm on a healthy engine requires a serious amount of injector flow for a TBI that only has 2 injectors. The higher the RPM the exponentially harder it is to get fuel to the engine since you have less and less time to squirt it in.

Its been a while since I have done all the math and datalogs, but if memory serves me the stock fuel injectors will run around 5-6 msec of pusle width across the board at WOT on a stock TBI engine. At 6,000 rpm you have 4 msec to inject the required fuel and stay under 80% duty cycle. I see an issue on anything resembling a stock injector/fuel pressure/pulsewidth at or before 5,300 rpm. Many later stock calibrations reverted to asynch at WOT.