Trying to Diagnose an Fuel Economy / Intermittant without Scan Data

So I've got a general question inspired by some trouble I'm having with my wife's car. Basically, I can read codes on the car ( there are none ), but not live data, misfire counters or anything except codes. I'm basically just working with a scope.

It's fuel economy is sub par and I can see intermittent misses occasionally as a hacksaw pattern in the O2 signal on my scope, but most of the time it oscillates normally. I've checked current ramps and secondary on all the coils ( COP ), looked at the two injectors I can reach without pulling things apart and the MAF and all looked good.

I kind of feel like I'm chasing my tail here, and I'm wondering both in this case and in general, is it ever worth trying to hunt down these types of fuel economy and intermittent misfire problems when you have no access to live data, misfire counters or anything else expect codes ( and with pre-OBDII cars not even that ), or is it just pissing in the wind to even attempt?

I had the same problem with customer complaint "bad fuel economy", nothing alse, at the shop 3 weeks ago ...
It was an old 1994 Hyundai ... or something like that ... preOBD
I wasn't able to read DTC also ... only a scope to read the one, 3 wire, o2 sensor that this car have ... nothing alse heheh ...

The point is ... that scope O2 reading and experience drive me to fuel system checks ...
and after some time the result was "a blocked fuel pressure regulator".

Customer stop yesterday to say "hi and thank you for the repair" cuz it was long time issue ...

For me, as a tech, ofc it was worth it to spent some time to repair it ... as a customer i am not that sure.

So my issue is I'm a fairly new guy, trying to start a mobile diagnostics business focused primarily on engine performance and no starts, which targets end consumers, not established shops. I want to charge a fixed amount for a diagnosis, and I'm afraid of getting dragged into things that make that not worth my time. You expect an occasional outlier that may take a few hours to figure out, but OBDII didn't come into force here ( Israel ) until 2003, and I'm debating whether or not to provide services for vehicles that I don't have a scan tool for. There are allot of common problems that can be figured out with just a test light and a scope, but on general performance complaints I really feel like I need access to at least codes and fuel trims to work efficiently. So that's kind of what's rolling around in my head.

With poor fuel economy on older vehicles the cause is often poor compression a cranking compression test may not show it up as all cylinders may be low due to normal wear and tear, but following up a cranking test with a leaking down test or old style compression test on at least one cylinder will give you a base line. Valve seat recession can cause a general misfire low power high fuel consumption (for example on any Honda that has reached 80,000miles/100000km).

Erratic misfires are harder to pin down without OBD2, coil packs are always suspect particularly if spark plug changes have been neglected and vacuum leaks can be an issue.

One thing l should add is manufacturer specific low cost or free software that will read pre-OBD2 cars is out there f you search the internet, VW, Ford and Fiat group I know are covered and I have seen various options for Toyota I don't know about USA manufacturers.

Yeah, I got cloned / pirated equipment for Toyota and Honda off aliexpress, and I found third party software and hardware for Nissan, and this site has allot of good stuff for German and other European vehicles back to the 90's:

www.obdinnovations.com/

I'll probably also get VCDS lite. You'd be surprised, but there are actually allot of old German cars on the road in Israel, go figure...

Andy.MacFadyen wrote: With poor fuel economy on older vehicles the cause is often poor compression a cranking compression test may not show it up as all cylinders may be low due to normal wear and tear, but following up a cranking test with a leaking down test or old style compression test on at least one cylinder will give you a base line. Valve seat recession can cause a general misfire low power high fuel consumption (for example on any Honda that has reached 80,000miles/100000km).

Erratic misfires are harder to pin down without OBD2, coil packs are always suspect particularly if spark plug changes have been neglected and vacuum leaks can be an issue.

I would think the "normal wear and tear" would show up on an old fashioned vacuum gauge as low manifold vacuum.

I don't think I've ever heard the term, "valve seat recession". Care to elaborate?

No kidding about erratic misfires being hard to track down without OBDII, that's kinda what motivated this whole question.

Thanks!

Valve seat recession was one of the issues caused when lead additives were removed from gasolines. The valve sealing area especially on the exhaust valve gets worn and eaten away by the combustion gas passing through it. As a result the valve gradually sinks into the cylinder head and the valve clearance (valve lash) is reduced and eventually will affect valve sealing and valve timing. This one of the major reasons hydraulic tappets are almost universal on all engines, in days gone buy hydraulic tappets were not used much by European manufacturers.