I officially dislike computer controlled cars....my 91 L98 starts, idles as soon as you touch the throttle, engine dies and will not start. The positive is I found a list of issues I've fixed...several great ideas from here, BUT....I'm throwing in the red shop towel. Ignition system completely serviced, double checked, all is good. Going to double check fuel system one more time and replace fuel pump relay. I have 50 lbs fuel pressure when cranking, 35 lbs with key on not cranking. Theft system??? Fusible links??? Throttle body?? Definately not going to dealer in Louisville KY where car was purchased, but probably will have to tow to Hunt and Knight Automotive, they have the actual body work of a C3 on the sign and I had them work on my 1986 Pontiac 6000 SE a long time ago. Thanks again to everyones ideas...my issue is I do not trust most mechanics, particularly after what I found from Chevy dealer service.

 

Probably looking at a sensor. I would start by using a scan tool and see if anything is out of the ordinary at idle, then see how the sensors respond to a rev.

 

Well there is a red flag. Are you SURE that is the result of your test? Or is that a typo? You should have ~45ish lbs Key on, engine off, OR running w/vacuum line pulled. and ~35-40 lbs idling w/vacuum line hooked up. As you open the throttle (when your symptom occurs, fuel pressure should rise as manifold vacuum falls, and under no circumstances, should the fuel pressure drop.


No, no and no. Theft system, it wouldn't even crank (starter wouldn't run). Fusible links, same, or it may crank but not start. Throttle body? Maybe a throttle position sensor...are you showing a check engine light? If so, you do not need a scan tool to diagnose. Use the paper clip method to draw codes and poste them up here.

 

I must have missed this part when I read your post.

That is odd. Is the Fuel pressure dropping down to 35 after initial prime, or is the initial prime only going to 35?

I wanna lean towards saying that it should be around 50 while cranking if the oil pressure is above 4 PSI, but I'm not 100% certain.

At this point I would investigate 2 possible problems. 1 that your pump is not priming enough during the 2 seconds that its on with KOEO. 2 that there is a possible clogged injector or non opening injector.

If I recall correctly, my fuel pressure primes to about the same pressure as it has when running with the vacuum line disconnected, if not more pressure. Both with a performance pump and a non performance pump.

 

Fuel pressure setting has nothing to do with oil pressure. Fuel pressure setting on a TPI engine is not 50 lbs, no matter what the operating condition. That is way too high. It's somewhere between 35 and 45 PSI.


Re-read his symptoms. His problem doesn't occur during start up, and one clogged injector wouldn't cause this symptom either. You are giving bad advice.


Not higher. Priming pressure and vacuum line removed pressure is the same thing; they are both pump running, no vacuum.

OP needs to re-check fuel pressure, and check for codes.

 

 

You know that while cranking the fuel pump runs prior to the oil pressure switch closing, right? IOW, the oil pressure switch is only used when the Fuel pump relay fails.


Here is his symptom:
my 91 L98 starts, idles as soon as you touch the throttle, engine dies and will not start
He said nothing about a rough idle (misfire)


Your fuel pressure operation theory is wrong. The purpose of the regulator (along w/sufficient flow from the pump) is to ensure consistent pressure regardless of injector dwell time. Pressure should be no higher at idle (w/the vacuum line off) than at WOT.

 

 

This is very easy to confirm in one of several ways.
1. read the FSM.
2. go out to your car, unplug the oil pressure switch see if it starts. It will start...proof that the fuel pressure relay is providing power to the pump
If the car had to wait for oil pressure at every start up, you'd have a lot of extended crank times, don't you think? Oil pressure switch is a back up.

Copy that, on the rest of what you wrote.

 

 

Correct. The ECM energizes the fuel pump relay and primes the system for ~2 second at KO. The ECM again energizes the fuel pump relay when it senses a crank signal (which comes from the distributor). This happens w/in 90* of crank rotation in an L83/L98 and w/in 1 degree of crank rotation in an LT1/4.

 

That is because most "mechanics" are part changers not diagnostic technicians. They are hard to find. I personally like the diagnostics a lot more than changing out the parts. Doing a lot of diagnostic work in anything but a large shop in a city (even then, has to be the right managment) is a money loss.

Back on topic, you say it starts and idles. Then you touch the throttle and it dies and will not restart. When does it not restart? Right away? Or is it ok after sitting for a length of time?

Did you test for spark during cranking when it does not start (spark tester)?
Injector pulse during cranking (test light can be used)? If you don't know what this is. Youtube, ditto for the spark test. Both are simple tests.

If you don't own those, they are like $10 a piece, maybe, get them. You cannot diagnosis computer controlled cars with NO tools.

If you think your 91 is bad.........LOL.........if you only knew about the current stuff and things coming in 5-10 years=Mind Blown. Not trying to be a jerk, just giving it to you straight.

As Tom said, any codes? If you don't have a scan tool, you can do it with a paper clip (do a search) or via the buttons on the Driver Info center (need factory service manual for that one).

How does it idle? Smooth or rough?
Any colored smoke out tail pipes?

The more info the better. This is how "technicians" diagnose vehicles with information from customer, observation, and testing.

FYI, IDK what your experience has been with dealer service but if it was with something more than about 10 years old....it would not surprise me. Most guys don't want to mess with old car and few car capable of doing so.

I fixed a 94 Nissan 300ZX the first time with a bad miss. No codes either. 1 bad fuel injector. It was SOB to replace. I hated every minute of that car....and it was a LOT of minutes, but it was fixed. SO many vacuum lines and plastic that turned to dust.....it was POS.

 

My one scanner shows nothing, the small one flashes thru the check engine light, one long flash, two short flashes and repeats, I would not be surprised if it is an injector, they were supposedly replaced at dealer before I bought car. Everything else they did has been a cluster. As soon as it warms up (-18 behind my shop this morning) I am going to double check fuel pressure and ensure fuel line is clear from tank to filter, diagnostics are a whole lot simpler on my '72 Triumph and '64 MG vintage race car, as my man Joe Walsh stated on his last album...'I'm a analog man living in a digital world'...I will also try to extract codes....Thanks again for ideas/suggestions....

 

I have spark when cranking, it will restart after car sits for a couple of hours then die again as soon as you touch the throttle. I agree with the service issue my car was probably older than the 'tech' who worked on it at Chevy dealer. I know for sure he did not know how to use a torque wrench when installing plugs, I guess that kind of a tool is too old fashioned these days. I will check for pulse on the injectors thanks again...