So I don't normally make posts but this problem has been troubling me for 3 years. The problem is the car has been consistently getting craptastic fuel economy since its inception. (20-25mpg with A/C on, 24-30mpg without.)

The Problems:
Black smoke
Hard to start after a heat-soak(at least 10 mins) and misses on 1-2 cylinders for 8-20 seconds afterward.
Bad fuel economy

These events occur in hot or cold weather.

I built the car 3 years ago from the ground up with the following parts:

New:
88-91SOHC ZC non-VTEC & pm6 ECU
DX tranny from Syncrotech
NGK Plugs, wires, cap & rotor
Fuel, oil, & air filters
OEM fuel pump
Coolant Temp, TPS, and Denso o2 sensors
Water pump, timing belt & tensioner
New wheel bearings and brakes all around
Valves were adjusted
Compression is 200ish psi on all 4
No Power steering
Exedy clutch kit

The car has a short ram intake, ZC header&downpipe(4-2-1) and 2.5" exhaust all the way through with no catalytic converter

My tires are always kept at 44 psi(sidewall max) and the car is very easy to push and roll from a stop. So there arent any stuck brakes or bad wheel bearings.

So why the bad fuel economy? The car blows black smoke at higher rpm but has a brand new oxygen sensor and TPS adjusted (properly?). I suspected the coolant temp sensor first and replaced it to no avail. Could this be the fault of the PM6 not being able to correct for the ZC or something? Maybe I have bigger injectors and I don't even know it?

Is there a way to test the fuel injectors or discern their size? A way to measure the voltage that the coolant temp sensor reads to ensure that it does its job? Do I just need to break down and get the car tuned?

I am completely at a loss at this point. I really appreciate any help in the right direction and will troubleshoot and report back with the fix if anyone has any suggestions.

 

Your mileage does not seem that bad. Black smoke could just be unburned fuel. Either your running rich, if you have a cat it doesnt work or the piston rings or going bad. The hard start issue may be head gasket issues since there are misfires on 2 cylinders. I would look in a Haynes or Chilton for the values but the coolant temp sensors should just be a resistance range to tell whether or not it is acceptable. The injectors with have a similar value as well. Size would probably be determined by flow testing or maybe markings on the injectors.

Do you have the ability to get the ZC ecu? I believe they have a bit more compression and this may help the engine run smoother all around.

 

MAP sensor can cause it to run rich, thats probably where you should check next.

 

Alright. I'm coming up on a full day off that I'm able to use to tackle these things...

So far I'm going to do the electrical checks on the coolant temp and MAP sensors. Is it possible for a vacuum leak to throw off readings on the MAP sensor? I do not have an erratic idle-- but my brakes do go hard fairly quickly after shutting the car down.

Maybe if there's somebody who also knows the correct vacuum diagram for the SOHC ZC intake would be willing to share? I do have blocked off vacuum ports on the intake that appear to have no home. This leads me to believe that I may not have hooked up the MAP sensor properly..

I have also found out through research that the most correct ECU for the SOHC ZC non-vtec is indeed the PM6. So I believe I am running the correct ECU for my setup.

If anybody else has any other ideas or things I should check while I have the time, please chime in. I will check them all at once. I really do appreciate the responses. I've done so much research and this is the only time I've ever come up dry on an issue.

 

So I measured the values on the coolant temperature and MAP sensors today and both checked out entirely with their resistance/voltage values per the FSM. I am still completely at a loss as to why this car has terrible fuel economy because both sensors are good.

I am only left suspecting my injectors... Any idea on how to tell what size they are? Also, would it be better for me to order a set of injectors off of ebay in good condition for the a6?

And would it be beneficial to see about getting the car tuned? It does technically have I/H/E and I forgot to mention it has had two valve jobs because of bad gas sticking and bending valves. Maybe this could help with power/fuel economy?

 

lol I never thought I'd come back to this thread, but my last fill up was for 19mpg. It's been getting worse. Tires are still kept at about 40PSI and no bearings or brakes are sticking. TPS tested spot on at 4.5v WOT. Car could definitely use an alignment now since my front tires are chewed up a little on the inside, so that's one thing to consider, but it really isn't in the budget right now. Also, I still live in central Florida and the lowest has been 66f since April or so. It warms up fairly quickly.

Since 2013, replaced:
FPR
Ignition coil
Cap/rotor/plugs/wires
igniter unit
Injectors
Radiator
Condenser fan motor
Raxles warrantied and refurbished

I just fixed my charging problems by cleaning every single ground on the car, adding another 4 gauge wire from trans to body, and wire wheeling the alternator surface and bracket mounting areas. It used to charge at 12.7 at idle, now it's at a straight 14v. My lights are also super bright again and the car cranks over like a raped ape. Should've done this crap years ago.

The car runs TOO goddamn good, it's super smooth and has some fun power to toy with.. I'm starting to think my gas is evaporating. I only fill the car once every 2-3 weeks, and each drive to work is about 7 miles one way with a good 2-3 cruising at 55. I bought a replacement gas cap from Amazon but it was for the wrong goddamn car. Do you all know if a plugged up original charcoal canister or some other broken evap line will jack stuff up like this? There is no reason in my mind that a SOHC ZC with a DX trans should be getting gas mileage like this. I'm not racing at every light.

Any help? I would really like this car to last for maybe another year, and 19mpg sucks ***. I forgot to mention in any other posts that when I first built the car it was consistently getting 30-35mpg, even with 1 mile round trips to work. It just keeps creeping lower and lower these days. I plan to do the following soon: Check ignition timing, valve adjustment, replace gas cap, possibly delete charcoal canister, alignment, check fuel pressure (Need that goddamn banjo bolt).

 

 

It's not really cold outside right now. Lowest was 66f overnight Sunday into Monday morning. It's been like 93-95f up until Sunday-Monday. The injectors were refurbished and tested for flow/pattern. I've also cleaned the throttle plate last year.

I've used Gumout 4 in 1 fuel system cleaner once or twice in the last 5 years, but not recently.

The black smoke still does appear at higher rpm. I would agree on running rich. I really need to check the fuel pressure but that elusive service bolt is tough to obtain.

I need to get under the car again to inspect the fuel tank lines. I do get a whiff of gas every once in a while, but I figured that may be associated with running rich.

 

 

I kind of doubt fuel cleaner is going to help much. My roommate might have a can of seafoam to use. Money's a little on the hard side right now, so I'll be checking the free stuff first like fuel lines, valve adjustment, clean PCV, ignition timing (If I can borrow a timing gun).

2013:
88-91SOHC ZC non-VTEC & pm6 ECU
DX tranny from Syncrotech
NGK Plugs, wires,
Honda Genuine Cap & Rotor
Parts house Fuel, Mobil 1 oil, & air filters
Genuine Honda fuel pump
Parts house Coolant Temp, TPS,
Denso o2 sensor
Genuine Honda Water pump, timing belt & tensioner
Timken New wheel bearings and parts housebrakes all around
Valves were adjusted
Compression is 200ish psi on all 4
No Power steering
Exedy clutch kit

Since 2013:
Genuine Honda FPR
Parts house Ignition coil & igniter
NGK plugs/wires
Genuine Honda Cap/rotor
Refurbished Injectors
Parts house Radiator
Parts house Condenser fan motor
Raxles warrantied and refurbished (Genuine MTF changed as well)

Denso is the OEM supplier for Honda O2 sensors of this year. 10w40 and no on oil pressure, the light works so whatever. Fuel is 87 octane, usually from Wawa. Don't know valve lash yet, need to check. PCV is a year or two old, but I haven't had my hands on it in a while. Wires were new NGK both times. I built the car, not the engine. It was purchased from the local importer, and refreshed with new ignition, fluids, filters, seals, and timing.

I did have to replace one of the lines on the gas tank back in 2015. It was dry-rotted and leaking. It was one that could be done without lifting the car up, but I couldn't tell you which one. Life was kind of dark back then.

 

Check your ignition timing for sure, If you can't get it into spec your mechanical timing might be out.

Did you ever check the line to your MAP sensor? If there's a hole in your vac line to the MAP your sensor will see much closer to atmospheric pressure and your ECU will think you're WOT.

 

Pull the engine codes.

Also check for a cracked exhaust manifold or an exhaust leak near or upstream of the O2 sensor.

 

I'll check for codes after work today. Might not be any since I just had the battery disconnected for an alternator swap. Appreciate the responses, ya'll. I'm hoping having good charging voltage again will help a little.

I did check the MAP sensor lines at the time, but not recently.

The ignition timing will have to wait since I'm having to sell my 2nd motorcycle to make rent, and I don't have a timing gun. HF had one with good reviews for like 30 bucks though. Now that I think of it, I'm pretty sure my dad set the timing without jumping the service connector 8 years ago, so we may be on to something. I'll likely have the cash for an alignment and timing gun mid-November since my roommate and fiancee are about to start their new jobs.

My gas tank is just about empty, so now would be a good time to crawl under the car and check out the lines I suppose.

I just replaced a cracked manifold on the wife's 96 LX last year. I didn't even think to check my car. I hope it isn't since I have the ZC 4-2-1 header.

 

If you recently disconnected the battery, run the engine for a couple of drives before checking for codes. Note that you will only be able to find codes if the check engine light works. Does it?

An exhaust leak could explain the running rich condition because it would fake out the ECU to think that the engine is running lean. In turn, the ECU would incorrectly increase fuel to the cylinders and lead to a running rich problem.

 

Check engine light works just fine. This is also the model and year for the red LED under the carpet. For some reason the charge warning light died with the old alternator, but works with the new one. Bulbs were good.

I have driven the car several times since replacing the alternator, so we can see if something is at least stored but didn't light up in the gauge cluster.

 

 

 

No stored codes on the computer. Filled up today for 23.8 MPG. It has been cool enough (60f-70f) in the last week or two to leave the A/C off in the mornings, though. This tank of gas also partly reflects the new alternator and fresh grounds, so I'm hoping maybe in two weeks when I fill again I can see how restoring my charging system helps the car.

I did get a heavy whiff of gas fumes outside of the car after work today. I let it run for a few minutes and had no drips, but I still suspect I need to drop the tank to check my lines. This obviously isn't going to happen soon now that I've filled it. I know we're onto something here but I just don't have the goddamn time when my tank is empty to look at it.

I appreciate the responses, ya'll. I will be interviewing for a salaried position with a 50% increase in pay and at least 10k in benefits, so I might just be able to grab a 2018/19 Hatch and start a build thread for a B20Z in this thing. I would be selling a Synchrotech DX tranny and two refreshed Raxles if that's the case and anybody wants dibs

 

Update: Ignition timing adjusted. Alright this was a good find, and likely a huge part of my issue. Fiancee and roommate finally have some paychecks coming in and I got some side work, so I bought a timing light, fully warmed the car up, sat at about 750-800ish rpm, and jumped the service connector.

This is what it looked like:

| | |_______|____|

The second from the right one is estimated where I found the timing set to. I moved it as best as I could in between the two far left ones, which is within the 18 +-2 degrees BTDC. I would venture a guess it was at only 10 or 12. The car picked up a faster idle, no longer has jerky 1-2 shifts, and seems to be running smoother. It started on the first crank instead of the 2nd or third. Before this, I'm assuming I've been at about 23-26mpg now since I've been driving with the A/C off. I'll fill up on Tuesday when I go back to work to give some hard numbers, then we'll see what the mileage is in another 2-3 weeks.

Still going to do the following, money permitting:

Valve adjustment
Aisin timing kit
Two new rear tires
Alignment
Clean reusable air filter
Replace original gas cap

Can ignition timing change over time? If not, then I've been driving the stupid car like this for like 8 years since I trusted my dad to do the work before I knew anything. I've been reading more on the timing being too retarded, so I'm really hoping this solves my rich mixture issues.

Edit: Map sensor line is good. Forgot to check exhaust manifold.

 

Update: Filled up last night for 28.1mpg. Looks very promising. I adjusted the ignition timing at about 230 miles on the odo, but I filled at 276, so I got a few miles in on that tank. There have been several drives without A/C though since we've been cooling off, so this could also affect my mileage. The car also doesn't smoke anymore when pushed fairly hard, which kind of blew my mind. I'm hoping for 32-40mpg when all is said and done.

I have the SOHC ZC, and the EF service manual pdf I have specifies 18 degrees for the SOHC. Whenever I search for ignition timing on the a6, everybody says to do 16 degrees. Thoughts?

 

I've got a few service manuals, they all reference 18°+-2º for SOHC PGMFI 1.6. 16°+-2° is for DOHC PGMFI 1.6.

 

Update: Filled up for 30.5mpg. Adjusted valves today. All but 3-4 valves were way out of spec, and most were tighter than they should have been. Intake was as low as 0.004", and exhaust was as low as 0.006". This is going on 8 years and about 70k miles since the last adjustment.

Florida's been between 45-75f the last couple of weeks so I haven't used the A/C at all. I'll be doing my timing belt job, coolant, and oil change in the next two weeks. I'm about to get the two rear tires replaced, then I can get the car aligned. I'm off for 12 days coming up, so I don't think I'll be able to give another mileage update for a month or so since I won't be driving anywhere really. Still hoping to get up to 35mpg or so, but we'll see how that plays out.

I did get the huge promotion, so I may very well be grabbing a 2018/19 hatch soon and setting this car aside for a B20Z swap. Not sure how the cards will land, but I'm still going to put a timing kit on this car in case I need to use it for another year or two. Not having a car payment is kind of epic.;

 

Update: Filled up yesterday for 32mpg. On Friday/Saturday I changed the oil/filter, coolant, timing belt, water pump, tensioner, valve stem seals, cam/crank seals, spark plugs, then readjusted valves. I can't believe I forgot to mention it had bad valve stem seals. This made for some bad smoking as per the usual timing of oil burning. Changing stem seals with the head installed is pretty easy on a SOHC, so hit me up if you need advice.

Between the crap ignition timing, out of spec valves, and oil burning, the plugs looked like this after a year:



They were replaced, for the record. Still have not done the tires or alignment due to money constraints, but I will likely update the main thread to include "ITT: Man neglects car and wonders why it gets **** gas mileage". I expect the car will only improve from here when I can afford the tires and alignment.

 

First thing i thought of was valve stem seals. I've had better success with marvel mistery oil rather than seafoam. I put in a few capfulls in every tank and it helps clean out the injectors over time. Also, the fuel filter is worth replacing (the one in the engine bay). Replacing that on my 270k mile CRX really cleared up a lot of odd hesitation issues. I'll also toss in a bunch of the mystery oil or seafoam type of stuff into my oil about 1k before an oil change. I took my intake apart and sprayed the intake manifold and intake tube with a ton of carb cleaner. That seemed to help me pick up additional MPG. Was hitting 44-46 on my CRX HF and after the carb cleaner and switching to Rotella t6, i'm hitting 50+ mpg.

Another thought i had is that your cat might be clogged? Might be worth a visual inspection. And your air filter... take it out and shake it like a rug to get all the crap out.

 

No cat. This car hasn't had one since I've had it. And the valve stem seal job has been a night and day difference. They were so bad that I suspected piston rings, but after this last weekend my car doesn't smoke at all.

Still need to wash the AEM reusable air filter. I did fully clean the intake manifold and piston tops in 2010 when the engine was fully apart. Fuel filter was done in 2013/14, but it really couldn't hurt to do it again.

Although I got a good promotion, my fiancee still has no job, so anything requiring new parts is once again at a hold. At this immediate point in time I have no further plans to do anything except bleed the brakes (fluid on hand), clean the air filter, and maybe clean the PCV valve.

The 32mpg figure is still without using A/C, and on shitty tires and bad alignment. I still hope it gets better than 35mpg but we'll see.