eutu1984

90 corvette does not have a MAF instead it has a MAP
http://en.wikipedia.org/wiki/MAP_sensor

JC316

Ah, thought it was 91 that they switched.

8a8mfh

$5 fuel related problem check: Get autozone brand brake cleaner, carb cleaner, etc. Take the air intake off, have someone turn the key while you spray something flammable into the throttle body (DO NOT put the little tube into the spray tip).

DO NOT stand directly in front of the throttle body.

You should be able to get it to run. I did this test on a '98 S10 with no fuel tank, fuel lines, or fuel rail.

No need to buy a fuel pressure tester yet, no need to buy anything except a can of something flammable designed to be sprayed into the engine.

Any chance someone put something in the gas tank?

aklim

I have a fuel pressure gauge. It isn't that it is kike a 1 use and throw thing. It can be used in future. Can't hurt to have one of those lying around. I do.

leesvet

EFI is 100% about precise fuel metering, pressures and delivery. Is there a point in making it fire and gurgle ? that does not tell you IF the Inj are actually opening/cycling, or if there is fuel IN the rails and how much. ALL those things have to happen at the right time and in the right amount,. otherwise, it's just gonna burp and gurgle or backfire...sorta like spraying something in the TB.

The pressure test gauge is the most basic and useful tool needed to do at home diagnosis. There are 3 things that you MUST know in EFI before going any further.....
1. if the pump & relay act right when the key is turned to ON

2. what the pressure is at key On and at engine idle

3. if the injectors are operating when/if they should.

You can learn ALL these things with a gauge and IF you know how to use what its telling you. It will tell you much more than just the pressure.....

IF the pump electrical circuit is operating as it should by activating the pump for the 2 sec 'prime' @ key ON, no crank, what that 2 seconds will generate as far as pressure is needed.
Should be 40-42 psi, and THEN, what is the pressure at idle, that should be slightly less, about 37-38psi. Steady

Then what the gauge needle is doing as it idles. That will tell you the injectors ARE cycling, if there is a filter problem and if the pump IS maintaining tension IN the rails.

This next thing is IMPORTANT and a bit tougher to 'get'...

You need to KNOW that the fuel inj are really cycling as they should. Watching the gauge needle while the starter is turning the engine over WILL show you the needle flicker and lose 2 or 3 psi as each inj opens briefly and the pressure will drop in 3 psi steps as the engine cranks over until it fires and runs.
This is because the pump is NOT replacing the fuel during cranking, so you can SEE the inj use the existing fuel pressure as the starter rotates the engine. The pump does not replace this fuel supply in the rails until the engine fires and runs. The engine MUST run on its own without the starter before the pump will run again. This is 'built-in' to prevent flooding.
The distributer sends reference pulses to confirm the engine is indeed running, and THAT is the OK signal (permission from the ECM) to resume fuel pump operation and continue in RUN mode.

Point being, after the 2 second prime from the pump, that little bit of pressure is what the engine uses to start up and get to the point of running and allowing the dist to send the required reference pulse. Prior to that, the starter is turning the engine over slowly, and the inj will open/close at the right time during engine rotation and each time they open they use a few psi of the "prime" or saved fuel pressure.
That can be seen on the test gauge as the needle drops downward about 3 psi each time an inj opens for a split second. This is why the fuel test gauge is probably the single most important and useful test tool to have for the C4 owner DIY guy. if you did not know this and how to get this info, you'd need a "noid" lite, test lite and/or a DVM to see if the inj were operating. And, the fuel pump relay and pump itself. The gauge will tell you ALL this good stuff...

Watching that gauges needle behavior tells you MORE than the psi. That will tell you if there is a pump OR filter problem just by how the needle moves. Flickering at idle, how long to recover when the engine is revved hard, how much of a drop in psi as the engine revs...? all things that can be learned from the gauge.

To work on ANY computer controlled engine you need the tools. Trying to use the old school methods that worked on 70's engines will sometimes do more damage than good and they do not point you at the source of the problem.
I would NOT want to be spraying solvents or brake cleaner into an intake that's got a MAP sensor anyway. Not a sensor that's getting a taste of that chemical.... it could ruin a diaphragm or other metering device in the MAP sensor...Brake cleaner is VERY destructive to certain types of plastics or compounds. Which ones? I dunno...you can always spray some on a few things and see what survives !
I totally ruined a very expensive remote control device
(several thousand $$) by spraying brake parts cleaner on some switches to douche them out when we ran out of electrical spray cleaner....The chemical dissolved the plastics and turned it into something similar to dust? the parts fell apart and dissolved the first time they were touched or bumped. BIG mistake ! Since then I try to use the right chemical in the right application....who can say what these 'flammable' spray cleaners do to O-rings in a fuel inj system? I'd pass on that trick. Brake cleaner is the Devil.

BTW, the other common suggestion of pulling the MAF plug off and using that to tell if the MAF is bad, is a fallacy as well. That does NOT tell you the MAF is good or bad just because it will now run without the MAF plugged in....ALL that does is force the ECM to run in OL and that could mean that ANY of the vital sensors are faulty, not necessarily the MAF. I have run mine half way across the country with the MAF d/c when I was traveling and needed to roll on home when the car started to run like crap and set codes...the fastest and easiest way to get it to run again IS to pull the MAF plug so its in OL and runs off the Cal-Pak in the ECM. That will get you home every time. Won't help you in figuring out what is wrong, but it WILL get you home. Avoid the temptation to drive all the time in OL. That might delay the repair cost and downtime, but it also means your engine is running out of tune and wasting fuel...since OL is a program in memory and not real time data it cannot run the engine precisely for the current conditions...close but not exactly. Mine shows about a loss of 3-4 mph hiway when running in OL for extended periods.

The $50 (or less) spent on the 2 books usually referred to as the FSM set, will save THOUSANDS of $$$ over the years in money saved on unnecessary parts, not to mention time.

IMO, the OP does indeed have a fuel problem and my personal experience tells me that proving the problem with a gauge and a couple other quick easy test WILL get him at the parts counter ordering the necessary things, and NOT a pile of stuff he does not need. The stinkin stealership should know how to do this, but again, they make their money selling parts, NOT solving problems.

Its shameful that a place like a Chevy dealer cannot properly diagnose ANY model Chevrolet made in the last 50 years..... That screams "parts changer" ......NOT mechanics or tech s !

aklim

After all is said and done, the injectors are junk. You can't tell spray pattern well without a flow bench. If dump the injectors and send them for cleaning every few years.

Tom400CFI

Actually the fuel pump does run during cranking. And rail pressure should not drop at all. If it is, something is wrong.

8a8mfh

It's a 5 minute way to tell if you have a fuel problem or ignition problem. Thats all I'm saying. As said earlier in this thread the mechanics are kids younger than the car. I can personally make an engine run with a spray can safely and long enough to rule out any other system except the fuel system. From experience.

What about the timing chain, plastic gear. I've seen them break and slip. It'll get spark but not at the right time.

The difference between my thought and everyone else, I'm trying to think of what it could be other than the fuel system.

Churchkey

Your 90 has multi tech injectors that are know to fail. The injection is not sequential it is batch fire which means if one injector shorts it can stop all of the injectors from working.

Ohm test the injectors they should read 14-16 ohms. Any that read lower than 14 ohms have failed.

aklim

Why even bother? You know they are junk right out of the box. Might as well replace them. Eve if it is reading 15 ohms, so what? It doesn't say if the spray volume or pattern is right. You are going to have to toss them sooner or later since you should be cleaning the injectors for that old a car and they are not worth the money to clean. So what to do?

Churchkey

He needs to bother BECAUSE he is trying to find the problem.
Has anyone else recommended checking injector ohms?
Multi tech injectors are not "junk out of the box" they have operated properly for many years. 21 years in my 90 before they became a problem due to the panther **** fuel available now.

aklim

I understand that but what to do? So you ohm them and they are fine. Doesn't tell me if the injectors are doing good. I had a set that ohmed out fine. Testing showed one collapsed filter basket in one of them and the rest were so so as far as spray pattern and volume was all over. So if you want to send it for cleaning and testing, why? You won't be keeping them anyways.

Fuel is a problem with the ethanol that eats away at the coils. It was designed for fuel to cool the core. Ethanol isn't that good to the windings. It is going to happen so you might as well change them while you have winter and not able to drive. If I got a vette with stock injectors, they will be in the trash can before the end of the day. It's just a time bomb waiting to go off at some unspecified time.

Tom400CFI

I think we're all aware of the disadvantages of multecs. but what have they got to with this thread? What did the OP say?
How likely is it that his injectors died while he was down town? Not very.

aklim

I agree it is highly unlikely that all 8 or more than 1 died simultaneously. Just thought I'd fix it while I am cranking on it anyway

leesvet

That is incorrect.

ref: FSM 6E3-C2 FUEL PUMP ELECTRICAL

I'll explain....

when the key is turned to ON, the ECM runs the pump via the relay for 2 seconds to "charge" the rails. If the engine does NOT start and run during that 2 seconds the pump will shut off and wait for the dist reference that says the engine IS running. Then the pump turns OFF until the engine starts and fires.

This 'charging' of the rails is called Pretension.

As you continue to turn the key, the pump has run for 2 seconds and turned OFF IF the engine did not fire and run. The starter is now in control of the circuit. This actually interrupts the pump circuit so it cannot run the pump again while the starter motor is engaged. Why? to prevent flooding of an engine that's got injectors opening under pressure but NO "run" indicator from the distributer as of yet.

It would flood in a second or two if the injectors were cycling AND the pump were sending more fuel to the rails.


Now, the starter is rotating the motor and the "fuel charge" that's in the rails IS allowed to enter the cylinders as the fuel injectors cycle. The slow cranking with the starter is using this fuel pretension to start the engine. If its stone cold the CS system adds more fuel, as the ECM permits.

This is the normal start-up sequence. happens this way every time, no matter hot or cold or how long since the last start-up.

The ECM can see that the starter is doing the work so it does NOT allow the pump to run again. As soon as the dist sends a reference signal AND the starter circuit is now disengaged, the ECM WILL then allow the pump to run and continue to charge the rails. The injectors are allowed to cycle as part of the start/run process. They must operate during cranking otherwise the engine would have no reason to start and run. It has sufficient fuel (pretension) in the rails to get it to wake up and rotate on its own. Instantly the dist sends the reference signal and its all an automated series of events from then on.

A faulty fuel pump relay can cause the pump NOT to run BUT the oil pressure switch should close and complete the 2nd circuit TO the pump as soon as the above qualifications are met AND there is 4 or 6psi oil pressure established.

So, the pretension in the rails gets used to start the engine with the starter motor and because it is cranking at a very slow rate of rpm, each pulse or cycle of the injectors can be seen on the gauge as a little bounce or dip in the fuel pressure. The longer the starter is cranking the motor the more you will see the fuel pressure DROP by 3 or 4 psi at a time when the injectors open briefly. You CAN crank the engine over long enough ( takes about 6-8 seconds IIRC) to use ALL the pretension and go to zero pressure, BUT the oil pressure switch usually gets a few psi to complete the pump circuit and let the pump run again, so going to ZERO pressure after extended cranking is rare but it DOES happen.

You can unplug the dist hot-wire and crank the engine over and SEE all this happening on the fuel test gauge. D/C the ignition so it cannot fire and run and the cranking time will show you the fuel being used at each injector cycle. IIRC, it will take about 6 to 8 seconds of cranking to use all the pressure stored in the rails. I may be wrong...its been a while. I actually need to go do some diagnostic work on mine, so I can take a look again and see how long it takes to crank away the 40 psi stored for start-up....IF your fuel system is in good shape. This is where a LOT of start up issues come from...a fuel system that does NOT hold that pressure will make for hard starts...hot or cold.

That is why the test gauge is able to tell you more than just PSI...if you know these small facts.

If the oil pressure switch is bad or faulty or d/c, you CAN crank on it long enough to use all the pretension and not get anymore fuel in the rails UNTIL you let off the key and hit it again, making the ECM run the pump for another 2 second "charge" or "prime" as people like to call it.

All the engine has to do to start is fire and rotate the crankshaft and dist drive at a speed the ECM will see as the engine running on its own without the starter. As soon as the ECM gets its reference pulse from the dist, it grants permission to run the pump as needed. Normal running mode, OL until it gets to CL temps and other conditions.

There is some of my opinion in this, but most comes from the FSM.
Bottom line, the pump does NOT run during cranking after the 2 second 'prime' ...until the engine either starts and runs OR the oil pressure switch closes.

leesvet

There IS the REAL possibility of a shorted injector that's shorting the whole bank. Ground one and you ground them all. I'd look at wire and get the DVM on the harness to search for a short in the inj harness. Do the grab/wiggle wire test. I'm sure one of those highly trained trade school techs can handle that........


Trade schools and Gov grants... Your tax dollars hard at work.
Pay your (my) money (tax money-gov grant money) get your certificate.
You're a circumcised GM tech now.

yesterday I wuz making fries....now I are a car technician and my new bes friends name is Snap-On.

Good lord, THAT debt will last longer than my Dr's student loan !

aklim

So we get over this mess and what? I'd still dump the injectors anyway. If I bought a stock vette and wasn't modifying it for a while, power wise, why would I bother with the injectors? I could take them for cleaning and testing but isn't that money wasted? If I was dumping them, why even waste any time with them? For $200, I get a rebuilt set that will last me for 3 years before I send them out for cleaning and testing again. While they are out, I can test the wiring if needs be. I'd also want to change the FPR regardless of how it all comes out since I am not a fan of a 20 yo piece of rubber.

When I buy a used car, I don't ask if you did the fluids. All fluids dumped. New belt regardless of what you say. Filters are changed. Plugs, cap, rotor and wires go too.

Tom400CFI

That ^^ is all wrong Leesvet. Sorry. but you need to re-edjumacate yourself. Fuel pump runs any time there is a pulse signal from the distributor. When cranking, there is a pulse signal. I don't think a single thing you wrote actually came from the FSM.

If you don't believe me, say so and I'll prove it.

EDIT:FYI, when cranking the motor, it's ingesting AIR...so the fuel injected under fp pressure, is proportional to the air...just as it is when running. Just like a carb has for the last 100 or so years.

jtc44

ding ding ding

leesvet

Actually, MOST of it is word for word from the FSM...I gave you the page, section and everything except the paragraph !


I can't make you comprehend those words though.....that's up to you and it looks like you do not want to open your mind enough to see what's being said here. The point has been missed? Whats AFR, carbs got to do with any of this?

I also gave the reasons and the HOW/WHY it is the way it is.
A statement without explanation is just an opinion. I am passing FACTS along to those interested in the FACTS. Those that would be interested in learning something, as I was.

The ECM manages the pump on/off. Until it gets the dist reference that its looking for, probably a higher enough rpm to indicate the engine IS running under its OWN power and NOT the starter, it DOES NOT allow power to pass thru the FP relay. That's a fact Jack. After that initial 2 secs...its done. No mas.

I also gave the other scenarios in which there could be power or NOT. Just because the dist is turning is NOT one of them.

But, of course My FSM must be wrong, of course. and I'm incapable of comprehending these words....so I will quote: (again)

DIRECTLY FROM THE FSM pages and sections that were listed.

'.....if the engine is NOT started in that 2 seconds (referring to the 2 sec prime time) the ECM will shut the fuel pump OFF and WAIT until the engine starts."

per FSM.


"...the ECM will shut the fuel pump OFF and WAIT until the engine starts"

I think its saying...No more power to the pump until the engine starts. Huh...I wonder why ? uhhh cause it might flood if it KEEPS on pumping fuel to an engine that's NOT firing and running yet>? hmmm. It has the fuel under pretension....that's plenty to start the engine. ALL it needs until it is running on its OWN and NOT the starter. THAT is built in. Go read the electrical section. JFC...

Food for thought....straight off the dollar menu.
This is TOO damn easy. Still not sure why the debate? Must be me.

I think they are trying to say that the ecm turns the pump OFF until the engine starts. AM I wrong? or is the FSM wrong?

That's still pretty difficult to "get". I keep hearing otherwise.

ya know Tom,
I really do not care why you want to debate something that IS pretty much etched in stone. Its etched in the FSM if nothing else...
My only goal was/is to help the Op, who probably gave up on this childish BS days ago. My info and FACTS are solid. I stand by them AND what is written in the FSM...both the emission section and the electrical book where the detail of the operation is even greater.

The whole ****in point of this was to provide a way of SEEING if the injectors are cycling in one easy step with a gauge attached. I've managed to learn a few things in my 30 yrs with C4s and L98 engines...
There is a TON of info available from a simple test gauge IF you know what to look for and how to interpret what you see.

I honestly don't feel the need to 're-edjumacate' myself when your description of the sequence of events is vague at best, and not fully detailed, I'm lost on that. I have no clue what a carb has to do with ANY of this subject and the OP's problem OR the FSM description of the fuel system and start-up mode.

It never ceases to amaze me how folks can go off on some tangent that takes the thread off in a direction that Mr Spock would have to study to find a way back....
If I'm wrong, please PROVE me wrong. Go scan and post the FSM pages that I supplied.

If it HELPS the OP solve his problem and the use of a fuel test gauge and its value in the diagnoses is worthless, then I'll be WRONG! who cares? I damn sure don't anymore. Its sad to me when you can't help someone without being challenged by someone that doesn't even have a dog in the fight...

its pretty obvious that you just want to debate something. I don't.


If your fuel pump runs more than 2 seconds when you turn the key,. something is wrong !


One more time....
SECTION:
drivability and emissions 6E3
PFI (port fuel injection)
sub-section C2 Modes of Operation pg-5

Paragraph titled: Fuel Pump electrical circuit

Bottom line,
The ECM shuts OFF the pump and WAITS for the engine to start if it fails to start in that 2 second "prime-time"...which most do NOT start in that brief period.

This debate is childish bu!!****. I'm done.
believe me, I KNOW how to copy and plagiarize. It got me thru school for many years....