nomad350Z

So I've been having some issues with my car recently. On a cold day, with a cold start, it starts and runs beautifully. No problems. But, once it heats up, it doesn't seem to want to run. On a hot day, it doesn't want to run even sooner. I've correlated that it runs better for longer periods of time when the weather is cold, and much less when the weather is hot.

The issues I'm having seem to be vapor lock; Sputtering, lack of power, inconsistent idle, stalling, the works. When the trouble starts, I have to keep the engine revving (above 2000). At a stoplight, I have to pop it into neutral, keep it revving, then slam it back in so that it won't stall. Even with this approach it doesn't make much power. On the other hand, above 3500, it runs like a champ.

The car had a 305 TBI that has since been replaced with a 350 TPI using an EBL flash system provided by RBob. The car does not display any error codes. THe only things out of order are a lack of a smog pump and the related controls, and a K&N Air filter bolted directly to the throttle body. I thought the lack of a cold air intake may be causing the vapor lock....

Its not a fuel pressure issue, why would it be when its temp related?

Could it simply be vapor lock brought on by no cold air intake? Or could it be a timing issue? Maybe too much retard?

Any ideas?

ibmtech

iTrader: (1)

I am kinda interested in the fuel pressure idea though. I thought the TBI engines ran alot lower pressure than the TPI. I might be mistaken on that though. I know roughly you want around 43PSI with TPI. I am not sure what the TBI pressure should be.

nomad350Z

I'm pretty sure its the same fuel pressure...after all, an injector is an injector, right?

Its got a fresh BBK 225lph pump, and no regulator until the one on the TPI base.....it ran great for a while, i.e. while driving it from NJ to Albuquerque NM. I don't think it likes the heat nor the altitude (5300 ft), two classic causes of vapor lock.

IF the timing were too far retarded, how would advancing it affect vapor lock?

ibmtech, with all those modifications to your motor, what kind of ECM are you using? do you have reprogrammable proms? How does it run as a daily driver?

BluFBdy

iTrader: (1)

this is great diagnosis and all but we forgot one small detail; Vapor lock is virtually impossible in our cars assuming that your fuel system is in proper order.

The issue with vapor lock was big when we had carbed motors with mecahnical pump that would use the diaphragm running off the cam to pump it, giving the lines roughly 4-6 psi, this didnt fill the lines all the way so on a hot day, lines boils, vapors are produced. HOWEVER, in this application you have an aftermarket pump and i believe our factory pumps run at 40 psi on average, lets just say there were vapors being produced, where would they go? there is no space in the lines for vapors to run or even be produced for that matter, therefore i think youre chasing a ghost with that idea.

Take a look at the pump, see what its pushing out as close to the pump as possible, then move up in the system and check it again.

I dont mean to bash your idea but vapor lock died shortly after the introduction of EFI, assuming the system is operating properly

what motor are you runing? Is it timed to factory specs? is it built? we need more details to help you out, picking and stabbing at an issue would take you years, instead of looking at whats wrong take a look at whats right, and start eliminating the problems from there

BluFBdy

iTrader: (1)

oh and TBI only requires 12-14 psi (i believe) i remember its very low compared to TPI but i fail to see how thats relevant because TBI and TPI use the same fuel pump its just regulated differently

nomad350Z

I'll check the fuel pressure. Like I said, it runs great on cold days (until it heats up after driving all day), so why would the fuel pump suddenly run differently in the presence of heat? Either way, I've put two fuel pumps on the car in 8 months and do NOT want to do it again. Fresh fuel filter (stock specs) at the time of the second pump installation (August '08).

I'm running a slightly built 350, standard 4'' bore and 3.48'' stroke. Motor is a Corvette L98, 113 heads slightly ported and decked, 9.5~10.0compression, Crower Hydraulic Lifter Cam, 204/210 duration with .485 intake .505 exhaust. Can't remember the LSA offhand, but the cam is designed for fuel injection in mind.

I can't remember the base timing set at the time of build, but I know it was right at 34* revving at 2600. This was determined using RBob's EBL system and a laptop. Then again, that is the computer adjusted advance, which does not monitor initial camshaft timing.

Only thing that is out of order other than this b!&@% of a problem is an exhaust leak. Don't think that would make any difference.

IF there is anything else you'd like to know, just ask. I'm baffled here. Thank you.

Oh, the car has never been tuned since the build. Currently its using a Prom that came with the system for a 5.7 TPI corvette. Does anybody know offhand what the cam specs on an '88 L98 where?

nomad350Z

any help here guys? I'm kinda dumbfounded.

BluFBdy

iTrader: (1)

well any part could fail with enough heat, im wondering if the pump is to blame but i dont want to tell you thats it just to find out its really not. i say take the car on a cold day and test the psi output when the car is cold and when its warmed up, then do the same test on a hot day, if the values arent exactly matched somethings up.

im wondering if the pump might be decreasing in performance as it warms up, kinda like its worn out? It may be newer but any part has a chance of a defect. I know how it feels ive done 3 pumps on my car because i got those defective parts

nomad350Z

Thats a good idea, but I'm gonna take it one step further. I want to check fuel pressure in the line before the fuel filter, after the fuel filter, and at the fuel rail both cold and warmed up.

Right not, I'm thinking it might be the fuel pressure regulator. Its original for a 305 TPI (engine is 350), and is in a prime location to get heated up real quick. Anybody ever heard of a regulator failing due to excessive heat?

BluFBdy

iTrader: (1)

even better!!

as far as the FPR goes id say it sounds possible, after all it is stock which was 20 years ago..... go ahead with the tests and we'll pinpoint it

nomad350Z

Just out of curiosity, how does teh fuel pressure regulator use the vacuum line inputed into it? Is it simply a vent, or does the vacuum pressure cause the spring in the diaphram to open and close, causing pressure to increase and decrease?

What I've noticed when these problems happen is that I have to apply throttle to keep the motor running. By doing so, I'd be increasing engine vacuum, which the motor seems to respond well to. Does increased vacuum cause a slight increase in fuel pressure at the regulator?

nomad350Z

ttt

BluFBdy

iTrader: (1)

its my understanding that when a larger vacuum is applied to the FPR it opens up more allowing more fuel to flow which would make sense in operaion, you hit the gas, it opens the plates on the TB, that takes the motor which is under vacuum and allows outside air (atmospheric pressure) in which puts a load onto the FPR, spitting more fuel.

seeing as it comes off the FPR straight to the plenum it makes sense, after all its also the source for the brake booster vacuum

Vacuum is higher under a load. So at idle the FPR should only spit out "X" amount of fuel and under load it should spit "Y" amount of fuel, "X" being a lower psi and "Y" being a higher number

nomad350Z

I thought that was the case. It hit me while I was pondering if vacuum was the culprit. I think we found the problem, but I'm going to rent a pressure gauge today and finalize it.

If it is the pressure regulator, I'm seeing prices all across the board for a replacement. Some adjustable regulators are at 60 from summit, others 130. What I'm thinking is, some of the regulators are only the adjustable caps, not the spring-loaded diaphragm, and others include both a new diaphragm and adjustable regulator cap. Anybody know which one is which? I'd like to get it all done at once....

TheGoodOld305

I belive youve got the vacuum backwords. You have more vac when at idle because the throtle blades are stopping the air causeing an increased vac. Throtle open= more air loose vac unlsess there is a restriction.

maybe this will help with understanding vac

http://findarticles.com/p/articles/m...g=content;col1
http://web.camaross.com/forums/showthread.php?p=4829362

BluFBdy

iTrader: (1)

yeah i wasnt entirely sure on the way i did it i get it confused for a second i didnt bother to think if it was the other way and now i feel dumb lol

in either case it still moves the diaphragm affecting fuel regulation

nomad350Z

So I took a pressure reading. At the fuel rail, it was reading 34psi and would not idle by itself. I had to apply throttle pressure to keep it alive. When I did, the pressure decreased (obviously, more fuel being consumed) and took a little while to re-stabilize at 34 again.

Checking the pressure aft of the fuel filter, I had some inconclusive results. I couldn't keep the car alive while under it checking the pressure, but the fuel pump is definitely functioning, sprayed me a few times pretty good. When I turned the key to the 'on' position, the fuel pump primed and stopped at around 15 psi. If I kept turning the key on and off (and letting the pump re-prime) it would continue to gain pressure.

Signs are pointing to a bad regulator. Any other input?

Street Lethal

iTrader: (16)

Wow, what a lot of misinformation. Your new TPI requires at least 40-47 psi of fuel pressure to run properly, so if your running the stock, or similar to, TBI intank fuel pump it needs to be changed to a pump designed for multiport fuel injection. Not all injectors are the same, especially in terms of TBI vs TPI. When you prime your engine (key on), the injectors do not pulse, so if the non adjustable regulator is reading only 15-psi, it's your fuel pump. If your positive that the new pump is designed for TPI, but is still reading low, it will still be your fuel pump, unless your regulator's diaphragm is leaking....

Street Lethal

iTrader: (16)

Are you still running the stock multec injectors....?

nomad350Z

Sorry for the misinformation about the TBI vs TPI injectors, rookie mistake. As far as the injectors go, I don't know. I bought the system from a friend of a friend. He said the car ran great when it was removed, and had some go-fast goodies on it. All I know is that the car ran great until I had my first fuel pump incident, explained below.

I'm positive the pump is designed for TPI, its the BBK version of the Walbro 225. Second one I've put in too....

I changed out the pressure regulator with no luck, still running EXTREMELY low fuel pressure (like less than 30). Thye pump is flowing, trust me its going....got doused pretty good by it yesterday. Its just not building pressure, which leads me to my next piece of the puzzle.....

The first fuel pump (aftermarket, not stock) that I put on lasted me for about 2 weeks, just long enough to get me from South Jersey to Moriarity, New Mexico when the car died suddenly. No indicators, nothing. Just dead. I did a pressure test and figured it was the fuel pump not building pressure. So I called up Summit racing kicking and screaming, and they sent me a new one.

When I pulled the pump out to replace it, I found that it was not the pump...it was a split in the fuel line inside of the tank. If anybody has ever done this, they'll know. The 5 inch long piece of rubber hose that connects the fuel pump to the steel fuel line inside of the tank.

What is happening now seems to be similar to then, except now I have slight pressure (enough to run). If there were a leak in the line, I would know it...unless it were inside of the tank again.

So two possible problems, one is a ruptured fuel line again. Two could be a fuel pump that runs, but doesn't build up enough pressure (which doesn't seem to be the case after seeing the rate of fuel flow from the tank).

Why does it keep rupturing fuel lines? Are they just crappy chinese rubber hoses used by holley (1st pump) and bbk (2nd pump) to save money, or do I have some other problem?

One thing I do know is that the pressure relief valve (mounted above the rear axle, the plastic little vent thing) is stock. I have to go to the dealer to get a new one, Autozone doesn't have it. If it weren't venting properly, there would be a vacuum inside of the fuel tank, whcih I would notice when taking off the gas cap (a hissing noise of air refilling the vacuum inside the tank, air going in not out), whcih I haven't heard.

Either way, looks like I'm pulling the tank out.....again.

Anybody ever have this happen?

Street Lethal

iTrader: (16)

That is definitely a problem, get rid of the rubber hose inside of the fuel tank and get a proper sending unit w/metal line from the pump, on up. In these instances, if the pump is good (like your confirming), you need to examine the other areas then, the first being that rubber hose. next of course would be the gas cap, make sure its on and functional. Next would be the regulator, you want to make sure the diaphragm isn't leaking. last, but not least, the injectors themselves. If the injectorsleak, you'll obviously run rich, and pressure will be lower than what it needs to be. First things first, get rid of that rubber hose in the tank....

nomad350Z

What is the alternative to the rubber hose? I have to use hose-clamps to secure it to the steel line above and the pump itself below. I was thinking of getting some better hose, rated for fuel and extreme pressure (like 300psi). Even then, I still need to use the hose clamps...what else can I do?

No fuel leaks that I'm aware of, and the regulator is brand new. Just installed it today. Never had a problem with the injectors, so all signs point to inside the tank.

I hate pulling tanks out of these things. One of the biggest engineering flaws by GM on these cars.

Street Lethal

iTrader: (16)

.... use compression locks, they will hold just fine submerged in fuel. Regular clamps should hold as well, but I would trust the compression locks. Here is what a TPI sending unit looks like, I just pulled it from the trunk;

nomad350Z

That makes much more sense. I still have the TBI sending unit in my car, and the length of metal line running down the the pump is nowhere near as long.

Thanks for the help, I think we just found our problem.

BluFBdy

iTrader: (1)

the pump itself is the same, checker, autozone and carquest all gave me matching p/#'s for tpi or tbi. The sending unit never even crossed my mind but why does TPI need the longer neck than TBI?

danziger

While it sounds like you may already found the problem/solution, my L98 had very similar symptoms and it turned out to be the injectors. The fuel system is VERY sensitive to injector performance...

Street Lethal

iTrader: (16)

What was the part number that they gave you....?

nomad350Z

So I pulled out the tank again. Checked the rubber connecting hose, no signs of damage, rips, tears, or leaks. At taht I swapped out the pump (again). Reassembled and it runs like a champ again! ......Or so I thought.

IT runs great again, but after awhile (90 minutes + continuous driving) it does the same thing. This time I downshifted to keep my RPMs up when it started acting up, and the problem went away. Weird? I thought so too.

What is the possibility of a Voltage spike decreasing the voltage to the pump? Has anyone ever had this issue? The fuel pump relay is fresh, could it still be bad?