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C4 driving me loopy

C4 Terry

Member
Joined
Oct 3, 2019
Messages
6
Location
Poole, UK
Corvette
1990 C4 Convertible
HELP...............!!
I have a 1990 C4 with the OHV 350 motor.
I bought it "cheap" as it had been stored for a number of years, very possibly because of the problem I have with it.
I now have the car running.
I replaced the fuel filter, injectors, ignition system, spark plugs, egr & position sensor, oil sensor/senders, water sensor/ senders, MAP.........and just about every other electrical doo hickey that can be replaced as a matter of service.
Apart from the engine being a little tired and rattly, it starts and runs great..............but with one problem.
Randomly, it will simply die.
Sometimes this can be after as little as a few minutes running, other times it's possible to get as much as 30 or 40 minutes before it dies.
No warning. No coughs or splutters.
Just like turning the ignition off.
The "service engine soon light" in the info center comes on and sometimes the "check gauges" lamp in the instrument cluster will also come on.

Sometimes it will re-start immediately and continue on.
Other times it simply wont start at all. It cranks but won't fire and when this condition occurs, the "sys" message on the LCD display will blink.

I've hooked one of those Snap-On scanners to this car and it simply says no codes present.

I'm now running out of things to change or test and spent a lot of time and money changing just about everything it's possible to change.
It's not possible to use this car as is...........it's simply too unreliable and is potentially dangerous as it has cut out on the freeway, which is just asking for a wreck.

Can anyone shed any light on possible causes as I'm close to giving up on this C4, which is a shame since otherwise it's a pleasure to drive .......until it stops.
 

Midnight 99

Well-known member
Joined
Nov 10, 2013
Messages
122
Location
Jefferson City, MO
Corvette
2019 GS Ceramic Grey A8 Conv. & 1969 Camaro SS
Have you checked your fuel pressure? Your fuel pump could be overheating and cutting out until it cools and starts to pump again. Just a thought.
 

C4 Terry

Member
Joined
Oct 3, 2019
Messages
6
Location
Poole, UK
Corvette
1990 C4 Convertible
Have you checked your fuel pressure? Your fuel pump could be overheating and cutting out until it cools and starts to pump again. Just a thought.

Yes Midnight. I've had a gauge on the fuel rail and the pressure is as expected and seems to remain stable.
No loss of pressure when the engine dies.
It's too sudden as well.
Past experiences I've had with fuel pump or pressure issues make the motor stumble or cough before dying.
This is like turning off the key.........bam. Stopped.
 

GWinter

Well-known member
Joined
Oct 2, 2005
Messages
52
Location
NW Iowa
Corvette
1990 conv. 6sp
did you replace the ignition module in the distributor ?
 

Delta98C5

Member
Joined
Oct 4, 2019
Messages
20
Location
Jefferson Parish, LA
Corvette
stock 98 C5 pewter 145K miles
You said you changed a position sensor. Was that for the crankshaft? I don’t know these engines so I don’t know if you mean that or possibly a camshaft position sensor. My dad years ago had an 84 thunderbird that would act like this except that it would run fine until you turned it off and then tried to start it up again. Sometimes it wouldn’t do it for days and then it might do it more frequently to make up for all of the lost time! It seemed to defy any kind of rational pattern. It turned out it was the crankshaft position sensor. You should probably clean your ground points up too if you haven’t already.


Sent from my iPhone using Tapatalk
 
Last edited:

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
Keep in mind that once the engine dies, if the ignition is still on the check engine light and the check gauges lights may come on and that's normal.

As for why the engine dies.

If you are seeing "SYS" blink on the IP and your Snap-On scanner shows no codes then the problem is not with the engine computer.

If you are going to go any farther DIY, you first need the Factory Service Manual for a '90. Do you have that? If you don't, you've already wasted a bunch of money "throwing parts" at the problem. The next money you spend needs to be for the FSM which will tell you how to diagnose problems that cause the IP to flash "SYS".
 

Robertwav1

Well-known member
Joined
Jan 7, 2003
Messages
812
Location
Ocala, Florida
Corvette
1993 Blue
Like Gwinter posted, if the car has an ignition module, replace it. The no code thing is because it's like turning the key to "off" position....something's turning off your ignition and the computer can't tell if it's you turning off the car or a circuit opening.
 

C4 Terry

Member
Joined
Oct 3, 2019
Messages
6
Location
Poole, UK
Corvette
1990 C4 Convertible
Thanks for all the input so far guys.

So............
I've found time to get back to looking at this troublesome C4 and after driving it round in circles in my neighborhood, I finally got the car where it had stalled out and wouldn't re-start ( much to the relief of my neighbors).
This meant that I could at last use diagnostic equipment on the car while the problem was more than momentary.
This is what I found.

Spark present. Good spark whilst cranking.
No injector pulse. Noid light does not flash.
No comms with ecu via my Snap-On scanner whilst in no start condition.
SRS light in info center ( which is now constantly on now that I replaced the bulb, along with seatbelt lamp) flashes multiple codes.
Check gauges lamp comes on in instrument cluster.
SYS is showing on LCD display.
No loss of fuel pressure when the car stalls out or when it's a no-start.
Fuel pump does not run (prime) when the key is cycled during no-start, although since fuel pressure is maintained, maybe it wouldn't.

My own feeling is that the ECM has developed a fault, as it seems dead in the no-start condition.
Before I replace the ECM, I'd like to check that power is getting to it. So, I found a pinout online of an ECM with plugs like mine but it has more than one which could be the power as they are marked as follows
"Ign. on power +12V" (B10)
"Battery Feed" (C1)
"BAT feed 12V" (D17)

So, I'm not sure which of these I need to check and under what circumstances they should be live or inert.
I'm guessing that these feeds are controlled by relays and / or fuses.
Does the SRS system/ controller have any control over power to the ECU..........?
In the event of it sensing an impact or crash, does it cut power to the ECM, or does it simply shut off power to the fuel pump / injectors / ignition.........?

Any ideas gentlemen (and ladies)...............?
 
Last edited:

Haldaddy

New member
Joined
Oct 17, 2019
Messages
1
Location
Des Moines, Iowa
Corvette
1991 Zr1 1996 convertible
I had a 1990 also. Had the same problems.....tried everything....finally it was the alternator wasn't charging....new alternator and off I went....no problem.....
 

C4 Terry

Member
Joined
Oct 3, 2019
Messages
6
Location
Poole, UK
Corvette
1990 C4 Convertible
That was interesting reading and gave me a couple of ideas.........but no dice with my C4

I wondered that too. I'd read that these cars were voltage sensitive. Checked the battery, even though it's pretty new.
Also checked the alternator, which seems to be charging just fine.
 

C4 Terry

Member
Joined
Oct 3, 2019
Messages
6
Location
Poole, UK
Corvette
1990 C4 Convertible
Working through the pinout I found, I did discover a poor ECM ground ( high resistance) but after running a new ground spliced into the existing ground wire, there was no change.
Pins C1 and D17 are hot at all times, except when there is a no-start. Then I'm just getting a bleed through voltage of, like, 1.2V.
So, something seems to be cutting power to the ECM C1 and D17 pins when this fault occurs.

Question is, what..............??

Do they get power from elsewhere before they run to the ECM, ......?
Maybe another control module or relay/ breaker.........?
Something that is fine when first started but develops issues when hot........?

I tried to look at a schematic in the Chiltons but it's simply not detailed enough.
Does anybody have a better schematic they can shoot me........?
Ideally I need a schematic from the 90 FSM.............but where the heck am I gonna find one of those in good old England........?
Looked online hoping for a PDF but no dice there.
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
You'll need to order a Service Manual from here in the States. In reading this thread, your car may have a number of problems with electronics. Without the diagnostic information in the FSM, you're left using a Chilton book, which is only one step above a catbox liner, and just throwing parts at the problem. Making matters worse is the Snap-On scanner you have likely does not support the CCM.

You can order an FSM for a 90 from Zip Products here in the U.S. It's 70.00USD. There is also a CD version of the Service Manuals for 88-90 for 30.00USD
 

Uhrlto

Member
Joined
May 27, 2020
Messages
6
Location
Reno, NV
Corvette
1986 White base model
HELP...............!!
I have a 1990 C4 with the OHV 350 motor.
I bought it "cheap" as it had been stored for a number of years, very possibly because of the problem I have with it.
I now have the car running.
I replaced the fuel filter, injectors, ignition system, spark plugs, egr & position sensor, oil sensor/senders, water sensor/ senders, MAP.........and just about every other electrical doo hickey that can be replaced as a matter of service.
Apart from the engine being a little tired and rattly, it starts and runs great..............but with one problem.
Randomly, it will simply die.
Sometimes this can be after as little as a few minutes running, other times it's possible to get as much as 30 or 40 minutes before it dies.
No warning. No coughs or splutters.
Just like turning the ignition off.
The "service engine soon light" in the info center comes on and sometimes the "check gauges" lamp in the instrument cluster will also come on.

Sometimes it will re-start immediately and continue on.
Other times it simply wont start at all. It cranks but won't fire and when this condition occurs, the "sys" message on the LCD display will blink.

I've hooked one of those Snap-On scanners to this car and it simply says no codes present.

I'm now running out of things to change or test and spent a lot of time and money changing just about everything it's possible to change.
It's not possible to use this car as is...........it's simply too unreliable and is potentially dangerous as it has cut out on the freeway, which is just asking for a wreck.

Can anyone shed any light on possible causes as I'm close to giving up on this C4, which is a shame since otherwise it's a pleasure to drive .......until it stops.

I read through this entire post, and I must say I'm very new to the corvette world. Having said that I have owned a lot of Camaro's and Firebird's. I had a similar problem on an 89 firebird and it turned out to be in the VATS system. The car would just suddenly stop because it thought it was being stolen and VATS would shut it down. I ended up having the vats system bypassed and the problem went away. Dont know if this is your problem but it is a thought.

Tony
 

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