Help - engine tuning problems

[martinlaw]

Hi,

Still have major engine tuning issues. The engine sounds rough, will backfire intermittantly, hesitates and has definitely lost power.

I have decided to go back to basics and test every detail and hopefully through process of elimination, can find the problem.

My Sport has a 450bhp 5.7 Chevy with a Edlebrock/Webber EFI and a optical distributor. The EFI is a basic system and it is setup via a hand held unit that came with the car.

A few questions:
1. What is the usual fuel pressure that the injectors work at?
2. Need to check the compression for each cylinder - what should the reading be in psi/bar?
3. The base timing is set at 10 degrees at idle which I understand is a good starting point, but when the management system takes over it leaps to 30 degrees at idle and I cannot get it to reduce - is this ok?
4. Does anyone know if there is anyone who is familiar with this system and has access to a rolling road?

Sorry about all the questions but as you have probably gathered, I am not very technically minded and this is a steep learning curve for me.

Cheers,
Martin.

[pb3]

A few answers, hopefully others can fill in the gaps/correct.

1. Typical fuel pressure will be 3bar which is 43.5psi.
2. Compression reading will be obviously affected by urh, compression ratio. Not sure how you can found that out and thus what to look for. But, when you test all the cylinders you can suspect any that are significantly different, someone may be able to suggest a suitable percentage difference to watch out for.
3. With your EFI system, the timing should be down to whatever has been calibrated, does it use a 3d map that you can view and check. I think TuxMan runs with a lot of advance at idle, sometimes it helps with idle quality.
4. Pass.

[crafty]

Get your hands on this book - it is an excellent reference for EFI systems and how they work.

www.amazon.com/gp/product/0760315825/sr=8-1/qid=1140170187/ref=pd_bbs_1/104-4126539-4407964?%5Fencoding=UTF8

[/quote]

[layada]

The 30 degrees may be coming from vacuum advance - remove it if you can and see if it still happens.

Sounds more like electrics than fueling to me. Suggest maybe checking the dizzy, rotor arm and coil.. assuming you have them

[builder]

martinlaw said:

The engine sounds rough, will backfire intermittantly, hesitates and has definitely lost power.

I have decided to go back to basics and test every detail and hopefully through process of elimination, can find the problem.

My Sport has a 450bhp 5.7 Chevy with a Edlebrock/Webber EFI and a optical distributor. The EFI is a basic system and it is setup via a hand held unit that came with the car.

A few questions:
1. What is the usual fuel pressure that the injectors work at?
2. Need to check the compression for each cylinder - what should the reading be in psi/bar?
3. The base timing is set at 10 degrees at idle which I understand is a good starting point, but when the management system takes over it leaps to 30 degrees at idle and I cannot get it to reduce - is this ok?
4. Does anyone know if there is anyone who is familiar with this system and has access to a rolling road?

Hmmm...backfire? Did this situation evolve over time, or was there an event that, after which, these symptoms started?

Here are a few things to check:

1. On the fuel pressure, locate the type of injector you have on the web and determine the proper pump characteristics based on your hp. Then, check the pressure and gallons-per-hour of your pump type. Verify the pump is delivering up to spec. If that's good, replace the fuel filter and measure again after the filter. If that's good, you may have an obstruction in the fuel rail or failing/plugged injector(s). All that said, see below for the order that I would perform these diagnostics.

2. On compression, it's more important that they are all uniform. It's hard to say what yours should be, but anywhere from about 45 psi on-up would be considered in the ballpark. The variation shouldn't be more than a few pounds. More than 10 percent variation could indicate other troubles.

3. Probably okay. 30 degrees sounds a little much. You will have to read the book on the management system, as I am not familiar with that setup. However, more information would be required. At what point does the system bump to 30 degrees? Does it ramp up to 30, or is it a quick jump in less than 300 rmp? Does the backfire start after the change in timing? Does the engine idle okay?

My strategy on engine diagnostics leans to checking the obvious first, replacing the inexpensive parts first, but doing whatever labor is required. These are the steps I would use check into your issues; spark and fuel first:

1. Check all plug wires for cracks, burns, and firing order. Replace wires if even a hint of discoloration from heat, or cracking, is present. Check the wire boots for condition and to make sure they are properly attached to the wire. Check all plugs for damage and pull them to look at electrode condition. Note the burn area of the plugs and compare to each other for uniform color. "Read" your plugs (example here). Replace plugs, if it's been a while. Check and/or replace the coil and coil wire.

2. As you pull plugs, you can do a compression test on each cylinder. It's a good idea. Do it for sanity, but a bad cylinder (bad piston, scoring, valves, rings) might also have other indications, like abnormal blue/white exhaust smoke. I check compression at every other plug change, which I do every 12 months or 10k miles.

3. If the plugs, wires, and coil are known good, and nothing has changed in your ignition setup, replace the fuel filter. It couldn't hurt to cut open the old filter and examine the contents. Doing so could provide indications for issues downstream.

4. EFI-Ignition System diagnostics. I think you are on the right path here. You've got to find, or become, an expert on your setup. First, chase all of the wires and make sure they are in good condition...epsecialy to the injectors. Then, if you can get a wire schematic, verify it's connected properly. See if there is a self-diagnostic for the ECU. You may have to connect a scope to verify injector pulses. If you can find them, you might get some help from the original engine builder as well.

Hope this helps, but my guess is you've probably tried all of this. Good luck and keep us posted as to the resolution.

Edited by builder on Friday 23 June 18:01

[davefiddes]

The idle timing doesnt sound a million miles out. I found my engine likes to idle around about 25 degrees of advance. The base timing is only used for starting where the reduced advance is a good thing because it limits the possibility of the engine firing against the starter...which means a quicker cranking speed (good). Once the engine has fired the ECU should pile on loads of advance to get it to idle cleanly. Often ECU spark maps will have a huge amount of advance at very low RPM which drops off to a stable valley at the desired idle speed. This means that the engine tends to gravitate towards the required idle rpm and avoid stalling. (try that with a dizzy!!!)

Getting a smooth idle is tough. Someone disassembled an old GM ECU from an 85 TPI Camaro and discovered that 30% of the entire code in the ECU was to do with getting a smooth idle. The Edelbrock system is about the same age and sadly not quite as smart...

You might find the megasquirt manual (www.megamanual.info) a useful guide. The specifics are obviosuly going to need translating to your controller but the principals behind getting a good tune still apply. Having a wideband O2 sensor makes the job a lot easier.

[burlguy]

I have had some experience with the above system. It sounds like your timing might be jumping around a little. Is there any noise coming from the top end can you hear any knocking clanging or anything out of the ordinary , of coarse besides the exhaust? The reason I ask this is that I have yet to see a blown headgasket cause back firing at idle. Sputtering/back firing is often either caused by faulty distributor signal/spark loss or valve train problem. I had in my case the same symptons and after tear down I found the hydraulic lifter went bad and lost its c-lip retainer resulting in a stuck open valve only while running at above 1100 rpms. The compression test did not reveal this. I could however hear a slight tapping from my valve cover.