Intermittent timing issue

**skp53** · 08-19-2023

I have a 93 Chevy p30 stripped chassis (step van) with 5.7 v8 tbi with at. The truck is stock except for egr delete. Sometimes after running a constant speed for 10 minutes or so, I get what I am calling valve rattle (machine gun like noise) when I come to a hill and press the gas pedal to the point of downshifting . I let up immediately and it goes away. If I don’t react quickly enough, I get code 43 knock sensor which I would think is to be expected. I was also getting 42, ignition bypass error, until I routed my spark plug wires away from the connections on the distributor. This engine is a jasper replacement that I have run about 8-10k and it has done this from the beginning. I let up as soon as I here it and downshift manually if need be to avoid it happening again. Sometimes I can drive it 2 or 3 days in a row with no issues. The distributor was put in new with the engine. I have also replaced the tps. Being that this is a stripped chassis, there are no bells and whistles on this one. I have also checked the wiring from the distributor to the ecm and checked the ecm for bad connections and bad solder joints. Doesn’t appear to be anything left but the ecm itself. Has anyone ever had a similar issue? Help is appreciated.
Btw, I have the interface and have downloaded tunerpro. The strange thing is (to me), it will not act up if I have tunerpro recording a file. If tunerpro is connected and up, it will still do it, but once I start recording, it will not repeat the issue.

**MO LS Noobie** · 08-19-2023

One thing I would try is a couple of tanks premium fuel, the higher octane will add some knock resistance. That's what it sounds like you're having. Another thing to check would be here ignition timing. Point to ponder is that is exactly why the EGR was put on the engines, to fight spark knock

**skp53** · 08-19-2023

Originally Posted by MO LS Noobie

One thing I would try is a couple of tanks premium fuel, the higher octane will add some knock resistance. That's what it sounds like you're having. Another thing to check would be here ignition timing. Point to ponder is that is exactly why the EGR was put on the engines, to fight spark knock

Thanks for the reply.
The timing is set at 0.
Maybe I should just put the egr back on the engine and have my chip flashed.
Seems strange that the problem doesn’t occur with tunerpro recording, but I don’t know how all that works.

**NomakeWan** · 08-20-2023

I'm not sure if it applies to your ECM, but many pre-94 GM ECMs will actually go into "service mode" if you connect a scan tool to them, which completely changes how the engine runs. I'd wonder if maybe your ECM is one of those that has to go into service mode in order to establish a scantool connection.

**skp53** · 08-21-2023

I’m not sure either. I can record a file with tuner pro and see the dashboard showing all the different parameters of the engine and speed etc.
Thanks for the reply.

**udidwht** · 08-24-2023

Always check and confirm fuel pressure when under load first thing with a TBI set up. Also check to make sure your ground cable/s are clean, clean, clean.

**skp53** · 08-26-2023

I’ll check on those. Thanks for the reply.

**tplasek** · 08-30-2023

I suspect your furl pump may be getting weak and loosing pressure under load. This causes the engine to go lean when under load, which will cause knocking. Letting off the throttle reduces the load on the fuel pump and lowers the timing. Both action would reduce the knocking. Fuel pressure should be around 13 psi when idling and under a load.

**skp53** · 08-31-2023

This truck had been parked for a while. I cleaned the tank, replaced the fuel pump, fuel filter, and put a kit in the throttle body. If I remember correctly, the fuel pressure regulator is in the throttle body, this was about 4 years ago. I’ll have to get a tester. Fuel pressure was mentioned before. Thanks for the reply.

**tayto** · 08-31-2023

did you use the correct submersible fuel hose on the pump? you can't use normal fuel hose, it will eventually swell and not hold pressure

**skp53** · 09-07-2023

The fuel pump was a complete assembly with the sending unit. I think everything was already
put together, just had to drop it in install gasket and lock ring and connect lines.

**skp53** · 10-21-2023

I have 12 lbs of pressure with key on and 17 lbs at idle and engine rev. The pressure drops to zero pretty quick after I turn the key off. This is a stripped chassis and the fuel filter where I connect the pressure gauge is just in front of the rear tires. Is there a way to connect near the engine? I could drive it with the engine cover off and monitor the gauge under load. The truck starts and runs fine otherwise. Would there be other symptoms if the fuel pressure dropped? Not really losing power.

**MO LS Noobie** · 10-21-2023

Look at your long/short fuel trims (called Block Learn and Integrator in your earlier systems) and oxygen sensor when it is acting up. Have you tried reinstalling the EGR? Typically EGR only operates at mid-range throttle, not idle or WOT. Most EGR commanded at heavy load/non WOT, same conditions you are having troubles. I concur with "service mode" comments on why it does not act up while recording data.

Odd that it is intermittent.

**tayto** · 10-22-2023

the pressure dropping to 0 psi on a TBI system after shutdown is normal. there's actually a bleed off orifice in the regulator. this is not like "modern" EFI.

https://marine-performance-parts.com...843505774.aspx

If your pressure is indeed 17psi, it is too high for a stock engine, i usually shoot for 13-14psi on stock stuff myself. you can make the stock regulator adjustable.