G'day everyone. Was wondering if I could get some help/confirmation reading my spark plugs. This is on my 1990 5.8L EFI.

Some more info for context. I have only the rear tank with a new Bosch pump, new fuel filter and FPR. Also new 9mm ignition leads, dizzy cap, plugs gapped to 0.055 and timing at 13 BTDC. I do however have 8 degrees of timing chain slack. Truck runs, starts and idles smoothly.

Symptoms are:

1) Poor mileage at around 8MPG
2) KOEO fuel pressure leaks down from 38-20 psi in around a minute.
3) I feel it doesn't have the power should, though I've not driven a similar vehicle to compare.
4) Only code is KOER 18, but I've narrowed it down to the IDM circuit which doesn't affect engine operation from what I read.

My question is regarding spark plugs 6 and 7. Would the colouring indicate leaky injectors? The discoloration is dry, not oily. I figure that would explain my mileage and low power. These plugs have only done around 1000km. Plugs 1-4 look basically identical to plugs 5 and 8.

I believe the injectors are original, but I did remove and have them professionally serviced a couple of thousand kms ago.

Another thought would be perhaps faulty ignition leads on those 2 cylinders?

 

There is definitely more disparity in color between those plugs than there should be and the fuel pressure should hold much better so something is going on. Does the oil smell of gas at all? That would tend to confirm a leaky injector or two diagnosis.

 

Thanks for the reply Conanski. Just checked the oil, didn't smell of gas. Also ran the truck for 5 minutes and pulled plug #6, it was not wet or oily. Kept a close eye on the fuel gauge over this time and it would intermittently bounce between 30-32 while idling. Engine wouldn't have been up to temp at that point though. It would hit 40psi at WOT.

I did hear a hissing though, which stopped when I pinched closed the vaccum line from the PCV valve to the manifold. But that would be a separate issue I guess. I suspect it's leaking from the grommet in the valve cover, as the plastic elbow seems fairly loose in there, despite it being a new Dorman grommet. Manifold vacuum for what it's worth was sitting steady at 20 at idle and quickly climbed back after WOT.

 

Was this a dual tank truck at some point and if so what was done with the fuel lines when the 2nd tank was removed?

 

Yeah the mid tank was removed when it was changed over to dual fuel LPG (propane for you guys?). I removed that, so all that's left now is two fuel hoses plugged with bolts, from what I can see. The remaining rear tank still has the vent hose going to the carbon canister and of course the fuel line to the fuel rail via a filter.

 

Try unplugging the vacuum hose from the fuel regulator. See if there is fuel getting into the engine from a blown diaphragm.

 

Thanks for the reply Dave. New FPR, new vacuum lines, and no evidence of gas in there.

I've also got new ignition leads but I'm going to check the resistance on them today, just be sure there's no faulty ones.

I did come across a random post on another English forum I think it was. Someone was talking about a valve issue causing a cylinder to run rich? Would that be viable do you reckon? I think he was suggesting if a valve wasn't sealing properly it would draw more fuel into a particular cylinder.

 

I also put a mechanics stethoscope on all the injectors I could reach, they all click at the same rate and volume and there's no fuel puddling on the manifold anywhere.

 

I've never been a believer in plug reading when running at part throttle and especially with unleaded. Unleaded running at wot will usually be either black or white with nothing in between. The electrode will be white yet the steel part of the plug will be black with soot that you can wipe off even. Looking into the exhaust port the valve will be white or nearly so, port walls will be black, header tube will be white where it is hot, black where it isn't.

You might want to verify the cam timing by putting it in split overlap. Either cylinder 1 or 6 can be used. With the cover off bar it over until both valves are open the same amount. Then look at the timing pointer. It should be at or very close to TDC. If it was running a tooth or two retarded it would reduce the manifold vacuum and economy quite a bit yet it may still seem to sound ok.

 

Interesting what you say about the white electrodes with black soot on the plugs Dave, that's exactly what my other 6 plugs look like. So that's pretty normal on unleaded?

I'm planning on a new timing chain once I scrape together the cash and time (and garage space) to do it. I measured 8 degrees slack there currently. When I do I'll check the valve/TDC situation, that would be very interesting.

 

You say this was a propane fueled truck when you got it? Have you changed the PCM? I'm not sure how they did these conversions WRT the PCM but a regular computer for gasoline won't work( runs too lean) so they either had to install a chip or swap in a custom propane spec PCM to fatten up the mixture. If you haven't been in there yet you should probably pull the PCM out for a look and if there is a chip on it pull it off.

 

Actually I recently changed the PCM to a reman unit (with matching number) from the US, when I was chasing a code 18. Prior to that I'd replaced the capacitor in the original unit. There was only one (capacitor) which I found interesting compared to all the research I did online from US sources. But our trucks over here seem to have a lot of older parts. Mine's a 1990 model but we have the interior you guys had in the early 80's, but right hand drive of course, plus the E7 heads weve spoken about. And don't get me started on wire colours in the truck compared to any other diagram or manual i can find...

But as far as the propane goes, I don't believe there was any chip. It's actually LPG over here and I've no idea on the chemical composition between that and propane, maybe its different? But as far as electrical stuff went with that, there was a manual changeover switch in the cab, two relays under the hood, and some cabling to the actual tank and regulator. All completely gone now.

 

But actually Conaski, I had heard that the trucks would run lean when converted to LPG. Maybe with the older carby Clevo's we had they could be adjusted lean, but then the new fancy EFI trucks arrived and all the dual fuel installers at the time couldn't work it out, so just plumbed it in to the new computer controlled system same as they'd done on the carby models.

The government had huge rebates and financial incentives at the time to convert to LPG, it was going to save the environment you know! But as soon as they stopped paying out in the early 2000s I think, people stopped fitting them, and started pulling them out of their cars. Service stations stopped supplying the gas as well. I live near the largest regional town in Australia, and there's only one servo there that still sells LPG.

 

Check your spout connection, if it is bad, it wont advance timing, when giving it gas to move the truck, if spout is working correctly, when engine is running, you can see timing jumping around, using a timing light, letting you know, the PCM, is controlling it, if spout is bad or unplugged, timing wont move and truck will be lazy.

 

Thanks for the advice Torq. I've had the wiring harness out twice in the last 6 months chasing down what I initially thought was a short in SPOUT circuit, and timed and re-timed it with SPOUT out. It's running pretty smoothly, I was just curious what those dry sooty spark plugs in only two cylinders would mean?