1985 C4 Suddenly No start
Thread Starter
Cruising
Joined: Oct 2023
Posts: 11
Likes: 5
1985 C4 Suddenly No start
Hi All,
Suddenly my C4 is not starting. Engine turns over and you can hear that there is combustion happening for fraction of second but then immediately engine dies. I got fuel pressure gauge and Im getting 35 PSI when pump is priming but when it stops it drops to zero in 2 seconds. I even managed to start the engine by pressing gas but it still idles really rough and pressure drops to 30 PSI. Tried to change pressure regulator which did not help. Is the pressure too low? I guess the next step would be pump replacement.
Suddenly my C4 is not starting. Engine turns over and you can hear that there is combustion happening for fraction of second but then immediately engine dies. I got fuel pressure gauge and Im getting 35 PSI when pump is priming but when it stops it drops to zero in 2 seconds. I even managed to start the engine by pressing gas but it still idles really rough and pressure drops to 30 PSI. Tried to change pressure regulator which did not help. Is the pressure too low? I guess the next step would be pump replacement.
Melting Slicks
Joined: May 2014
Posts: 2,895
Likes: 592
From: New Port Richey FL
Try taking the vacuum line off of the regulator that will max it out and you should be in the 40s also pay attention if fuel comes out (shouldn't) also pull a Fuel sample from the bottom of the tank. Check for water.
Just a side note on the pressure, Last fuel filter?
Just a side note on the pressure, Last fuel filter?
Thread Starter
Cruising
Joined: Oct 2023
Posts: 11
Likes: 5
I think it gives 35 also without vacuum, I was running the pump with bypassed relay for testing so the engine was not running and got 35 PSI. No fuel from the regulator vacuum line, this seems correct. Regarding the fuel filter I changed mine for new couple months ago, so this should be also correct. What noticed that the pump strainer is in bad shape, falling appart.
Melting Slicks
Joined: May 2014
Posts: 2,895
Likes: 592
From: New Port Richey FL
I think it gives 35 also without vacuum, I was running the pump with bypassed relay for testing so the engine was not running and got 35 PSI. No fuel from the regulator vacuum line, this seems correct. Regarding the fuel filter I changed mine for new couple months ago, so this should be also correct. What noticed that the pump strainer is in bad shape, falling appart.
Pro
Joined: Jun 2001
Posts: 509
Likes: 5
From: Edmonds WA
Hi All,
Suddenly my C4 is not starting. Engine turns over and you can hear that there is combustion happening for fraction of second but then immediately engine dies. I got fuel pressure gauge and Im getting 35 PSI when pump is priming but when it stops it drops to zero in 2 seconds. I even managed to start the engine by pressing gas but it still idles really rough and pressure drops to 30 PSI. Tried to change pressure regulator which did not help. Is the pressure too low? I guess the next step would be pump replacement.
Suddenly my C4 is not starting. Engine turns over and you can hear that there is combustion happening for fraction of second but then immediately engine dies. I got fuel pressure gauge and Im getting 35 PSI when pump is priming but when it stops it drops to zero in 2 seconds. I even managed to start the engine by pressing gas but it still idles really rough and pressure drops to 30 PSI. Tried to change pressure regulator which did not help. Is the pressure too low? I guess the next step would be pump replacement.
Thread Starter
Cruising
Joined: Oct 2023
Posts: 11
Likes: 5
I checked all injectors for leak with camera they are OK. So I bought and installed new fuel pump. I think the pulsator may caused that pressure drop, with new pump there is just straight tube. Now the fuel pressure does not drop when the pump is not running. But still had issue start, so I have bumped the idle screw. It start and runs fine, but I still think I have not fixed the root cause. Guess the IAC might be the next thing to check. Also to add some info my 85 C4 has only around 8k miles, but it was neglected in recent years and sit outside. So there is not much ware but some surface rust and oxidation.
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
I checked all injectors for leak with camera they are OK. So I bought and installed new fuel pump. I think the pulsator may caused that pressure drop, with new pump there is just straight tube. Now the fuel pressure does not drop when the pump is not running. But still had issue start, so I have bumped the idle screw. It start and runs fine, but I still think I have not fixed the root cause. Guess the IAC might be the next thing to check. Also to add some info my 85 C4 has only around 8k miles, but it was neglected in recent years and sit outside. So there is not much ware but some surface rust and oxidation.
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Thread Starter
Cruising
Joined: Oct 2023
Posts: 11
Likes: 5
I just replaced IAC and it runs more or less the same. I was also playing with that screw that sets the low throttle position so no it starts to idle at 900-1000 rpm but after couple minutes it goes down to ~600 I'm not sure if it is how it should be or if it was just tuning of new IAC. It still is not clear to me how all these things work together. But still even that now the car does not stall the idle is rough. What is awkward is that also bulbs in the interior and dash are dimming / blinking in sync with roughness of idle. Would actually like to understand why there is voltage ripple when the battery is fine and alternator seems also doing well. Voltage on dash is OK.
Le Mans Master
Joined: Jul 2004
Posts: 6,228
Likes: 698
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
I just replaced IAC and it runs more or less the same. I was also playing with that screw that sets the low throttle position so no it starts to idle at 900-1000 rpm but after couple minutes it goes down to ~600 I'm not sure if it is how it should be or if it was just tuning of new IAC. It still is not clear to me how all these things work together. But still even that now the car does not stall the idle is rough. What is awkward is that also bulbs in the interior and dash are dimming / blinking in sync with roughness of idle. Would actually like to understand why there is voltage ripple when the battery is fine and alternator seems also doing well. Voltage on dash is OK.
Thread Starter
Cruising
Joined: Oct 2023
Posts: 11
Likes: 5
Tried disconnected MAF no change. I mean now it actually starts, still the idle is not great but it does not stall. Maybe I will just accept how it is. Also this is my first time having corvette so I have no idea how the idle should look like maybe Im too sensitive.
Thread Starter
Cruising
Joined: Oct 2023
Posts: 11
Likes: 5
Ah, ok. Strange thing is there was no cap on the screw. I will see how the car runs. Today I noticed metal clunk from rear, diff I guess, when start to accelerate after braking with engine. So this seems like more important right now.
Edit: Also I see I should set 450rpm for idle this seems really low.
Edit: Also I see I should set 450rpm for idle this seems really low.
Team Owner
Joined: Jan 2003
Posts: 20,322
Likes: 249
From: Summerland B.C. Canada
Melting Slicks
Joined: Nov 2016
Posts: 2,256
Likes: 3,082
From: So Cal
2023 C4 of the Year Finalist - Unmodified
I own a 1985 since new. I had EXACTLY these symptoms in my car on a couple of occasions over two days of a 1,500 mile road trip. The dash displays went crazy, missing segments, gibberish numbers, and rough running.
It was bad connections at the battery. It cranked and started normally, but I guess something was resistive, and the "capacitance" of the battery was not connected to absorb electrical noise or ripple. The dash and ECM were affected.
Remove the cables and clean the connections and bolts. Also check the cables for bulging near the battery ends that might indicate acid intrusion under the insulation that turns the copper to green dust. If any doubt, flex the cable sharply within a few inches of the battery connection. If it goes limp, it is corroded internally and just broke. Replace it.