Does a larger TBI require higher fuel pressure?

[crowballs]

I have a 670cfm Holley TBI installed in my 90 K1500 350. I recently checked my fuel pressure to troubleshoot idling issues and it was at app. 10psi. That is within factory specs, good to go right, but then I thought of a question, would a larger cfm unit require higher fuel pressure, as I assume it also has slightly larger injectors. Would any of ya'll be able to help me out with this one?

[bwood]

I believe the fuel pressure would be the same, the ECM would need to fire the injectors longer.

[TennesseeZ]

Seems to me it could be either way. Stock pressure and longer pulse, or stock pulse and more pressure should both result in more fuel in the engine. I think.

[ChevyTech]

CFM is Cubic feet per minute. It is a measure of air flow not fuel flow.

The TBI fuel pumps are capable of flooding a 454 so it can feed a 350.

More fuel does not necessarily mean more power. Too much fuel will cause a loss of power

[scotts62]

Wasnt there a thread about putting a 454 TBI on a 350 and what all you had to do to make it work? Just wondering, i thought i remember reading somewhere..

[ChevyTech]

Originally Posted by scotts62

Wasnt there a thread about putting a 454 TBI on a 350 and what all you had to do to make it work? Just wondering, i thought i remember reading somewhere..

Yes!

[crowballs]

Thanks for the input gents, I'll dig around for that 454-350 post. Like ya'll said, it could be either way, longer pulse or higher pressure. Both make sense to me. I will tell you what I dig up.

On another note, yes it does mean a higher air flow, but without an increase in fuel to compensate for what would suddenly be a much leaner mixture.....that is what made me pose the question.

[jb1016]

I was just looking at that Holley TBI in the Summit catalog tonight. How have you liked yours? Did you notice much difference over stock?

[crowballs]

The info in the 454-350 threads was inconclusive to my specific question. But, I did do some digging around and on the holley website found the "insructions" for installation. Holley stated that the 670 requires only 11psi. That is pretty close to the stock range, but not dead on. Just how much would one pound of difference make? I did pop a code 44 about 2 weeks ago while sitting at a long stop light. Maybe they are related.

JB, the holley was installed on the truck when I bought her, but they also handed me the still good stock tbi. Soon after buying her, a few years ago, she has been giving me annoying running issues. I did put the old tbi on there out of frustration once, and if my memory serves me right the difference is there but it was only slight to moderate (that is the holley was a little stronger). Due to the present running issues, I can't give you any more details than that boss'n. Sorry.

[crowballs]

Scotts62, your pink name signature made me LOL. Witty, but a good idea. Maybe in time...

[70BLAZERX2]

I had a 1993 roadmaster 350 TBI i put the new "RAMJET"350 ROLLER CAM IN IT,was a very lopey idle..and would hesitate going up the road,I took and changed the fuel pressure spring in the reg,and made it so i could adj. the screw on it,(fact. has it welded) and turned it 1/2 turn to the right,And it was right after that..if you google "GM TBI FUEL PRESSURE SPRING" It should get you where i got mine. And it was running about 12psi..just needed a little more..

[crowballs]

I'll probably take you up on that idea 70BLAZERX2, but I am going to investigate one more issue before modifying my regulator. Hell, what am I saying, I probably still will. Doesn't sound too hard. I'll tell you how it goes when I get there.

On that note, does anyone know what the specs are on the EGR solenoid on this thing, I've got a haynes, but on that particular sensor it's alomost useless.

[ChevyTech]

Originally Posted by crowballs

...On that note, does anyone know what the specs are on the EGR solenoid on this thing, I've got a haynes, but on that particular sensor it's alomost useless.

It is not a sensor, think of it as a control valve that is turned on and off by the ECM.

Make sure no vacuum is getting through the EGR solenoid at idle.

[crowballs]

ChevyTech: "Make sure no vacuum is getting through the EGR solenoid at idle."

Did you mean test the solenoid for vacuum post solenoid, pre egr valve? Not sure I'm approaching this one correctly.

Well, after pluggin the line prior to the solenoid, I found no difference in the running characteristics and she also didn't pop a code, I thought this was odd. Still has an occasional hard acceleration backfire through the intake off low speed take-off, not often though. If I'm not mistaken this test eliminates the solenoid as the issue. I think I will try out the regulator mod tomorrow night and see how she runs. That is my last friggin' idea on my continuing idle/low end hesitation issues. If you guys have any other ideas I'm open, because I am out.

[ChevyTech]

Did you mean test the solenoid for vacuum post solenoid, pre egr valve?

Yes

Usually the fuel pressure reads on the high end of the specification if the pump is strong. Many times if the reading is 9 or 10 PSI the pressure actually drops off under some driving conditions.

Specific criteria / driving conditions must be met for a code 32 to get set.

Some people rarely hit the criteria and can drive for a week before setting a code 32 even with the EGR unplugged.

[70BLAZERX2]

go here..

http://tbiparts.com/index_links/TBI-AFPR.pdf

[scotts62]

Originally Posted by crowballs

Scotts62, your pink name signature made me LOL. Witty, but a good idea. Maybe in time...

Good least its a good laugh

[stsalvage]

I have a book for the 1987 TBI injector its supost to be between 9-13 pressure and they have a 700 tbi in here and it uses 9-13 PSI i will send you 2 messages with pictures showing what you have to do and what book you need to buy to figure this crap out ok

[stsalvage]

here is more pictures of the book you need to look for on Ebay hope this helps this book is choped up about every TBI that was ever used on the 4cycle up to the big block 502

[crowballs]

The problem is solved! Now on to the next one. Here is the skinny on everything that I can remember concerning what the problem was and the process that led to the solution. In the end it was you guys with the good gouge, particularly ChevyTechs persistence with the fuel pressure throughout this forum. Sorry it's so long!

In the beginning:

I puchased the chevy with the 670cfm holley on it. Within a few months the truck started to show a strange idle issue. She would crank and idle half the time at 1000rpm and then slowly fall down to 300-100rpm and bounce back and forth between 100-500, sometimes dying. Also, this erratic idle was intermittent, seemed to act up the worst in moist/cold weather. This is extremely frustrating with a 5-speed. I was new to the tbi units, ended up with all kinds of troubleshooting fun, the tps on a brand new tbi was bad. That helped with some of the idle/acceleration issues, and I had to take her in to a mechanic for that. He also checked my timing and reset my idle to 800 in the ecm. He told me once the TPS was replaced that she should be fine. It was summer and the problem seemed solved. Winter came. It wasn't. Same issue continued to resurface just whenever she pleased.

2yrs passed and the problems continued, but she was drivable. I was more concerned with a hard lower rod/crank knock(bought some time by replacing the bearings) and a leaking(jb weld fix)/rebuilt tranny.
Well, I got a paint job on the truck, and when the bed was off I went ahead and replaced the sending unit/fuel pump/filter. On the original the return line was rusted in half. Thought that would help with the problem. Wrong. Note that I had yet to be told or even think that the fuel pressure itself could be the issue. I started to trace out the issue. IAC was the first thing to be checked, I thought it was an intermittent fail, so I replaced that temporarily with a new one. The same exact results. Both the new and the old tested within specs. TPS..good to go. EGR...shot. Replaced it without any significant performance change. Distributer...somewhat faulty, the pickup coil was out of specs cold. That got me to pull the dist., I read about the weak magnet issue and stuff and checked that out. I could free spin my dizzy and it would go for a few turns(shouldn't do that)...replaced the unit. By the by, if you have a factory distro, take the time to pull it and check it out, it is amazing and sad that GM put that grade of engineering in their cars. Well, with the problems: erratic idle, low end power loss, occasional backfire, this should fix it right. Wrong.

In the end:

The final straws were the EGR solenoid and the fuel pressure. Tested the solenoid..good. Ordered a fuel pressure tester offline, did some home engineering, hooked it up. Tested 10psi. Good right? For a stock tbi yes. This holley required 11psi. I'll state this now, but I didn't know it then, 1psi makes a hell of a difference. I drove it around for a while thinking that the pressure was good and just thought that my timing was off or something. I rechecked every other thing and it was good, all my sensors to my firing order, all good. 70Blazer mentioned the regulator mod. Screw it, nothing to lose. I set everything up(by the way, holley made that adjustment screw hard to get to initially, but once you get to it, it sits on top easy as hell to get to and adjust) and noted that my initial fuel pressure was 9psi? Began tighting the screw in to raise the pressure and the results were noted within 1 1/2 turns. Brought her up to 13psi and have yet to have an idle/"bad" hesitation issue. IAC is able to do it's job with any loads(a/c, power steering, etc.). I am almost scared to say I think the problem is fixed. You wouldn't think she was 20 now.

Note: I found a slight hesitation in my take off, checked my dizzy/crank slack, found there is some timing chain wear and advanced the timing 3 degrees, will find out how that works later. Also, I'm gonna get some pics of the regulator mod and my engineering with my fuel pressure tester. I found it to be a very cheap and very simple work around.

Thank you gentlemen for all your help! I couldn't have done it without it! FUEL PRESSURE! HOOYAH!

[70BLAZERX2]

Your welcome..The wagon was doing all that crazy stuff and it did fix it..Glad you got it going..

[ChevyTech]

Glad you have stuck with it and have been successful, and thanks for sharing what you found.

I have no experience with the Holley TBI units but from what I have read they do not do well with low pressure.

Are you running a stock ECM with your Holley TBI?

NOTE: Often vehicle that have low fuel pressure end up having a weak pump or leaking connection hose in the tank. Hope you are lucky and it just needed a tweak.

[crowballs]

Originally Posted by ChevyTech

Are you running a stock ECM with your Holley TBI?

NOTE: Often vehicle that have low fuel pressure end up having a weak pump or leaking connection hose in the tank. Hope you are lucky and it just needed a tweak.

Yes, it's a stock ECM. When it starts getting to changing ECM's or flashing PROM's I am quite ignorant. You think I might look into one of the above?

As for that pump, the ground at the sending unit broke, so I had to "make" it work. I think that it might not have a great connection at the pump maybe lowering her voltage below desired amounts. But from what I remember, the lines in the tank all looked good though. I'm quite sure that will be another project sooner than I would like.

[ChevyTech]

Yes, it's a stock ECM. When it starts getting to changing ECM's or flashing PROM's I am quite ignorant. You think I might look into one of the above?

I would stay with a stock ECM unless you have mastered computer controlled fuel systems and are ready to spend hours building fuel tables to get it to run well.