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CA SMOG FAIL: High HC @ Idle Only

Mylesofsmyles

A Member
Joined
Jan 18, 2010
Went to get the car smogged today. 130 miles to smog station, and I didn't baby it, so the cat was hot. Car drove great and boosts confidently.

Backstory:
Colleced this car in October, 1992 Volvo 740 Turbo Wagon. It wasn't running, 460k miles on odometer. Drove it 1400 miles home, took it apart, serviced most things. Head didn't come off engine this time, but I cleaned & resealed all else.

See smog results below.

My head scratcher: Last week, I flushed the oil after getting it running again. Didn't know then, but I had a hole in CBV, and put 30+ miles of "shakedown" driving on the car.....would that "pig rich" scenario have loaded up the oil with enough Hydro Carbons to fail my idle test?

At the smog shop, we pulled plugs, and they generally looked fine.

PS, after test I had a CEL 1-3-1 which I believe would be bad RPM signal. But the car never stalled....not even on the 130 mile trip home

What would throw off my hydro carbons at idle like that? Also not the "elevated" CO%


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HC is raw fuel. Misfire, rich, or weak cat could do it.

Are you following the Philski thread?

You want properly flowing injectors, all fresh ignition components, properly working MAF and Oxygen sensor. If all those all in line, throw a cat at it and you will likely pass.
 
HC is raw fuel. Misfire, rich, or weak cat could do it.

Are you following the Philski thread?

You want properly flowing injectors, all fresh ignition components, properly working MAF and Oxygen sensor. If all those all in line, throw a cat at it and you will likely pass.
I will read Philski's thread too, to see if there's something to learn.

All ignition components new, save for Bosch Coil which seems fine. I can only assume on MAF and 02 sensor....maybe'll I'll recover details on how to test them. No clue on injectors. I replaced their o-rings and pintle caps when I refurbished everything.

The idle SOMETIMES stumbles...like a vacuum leak, and I can't figure out what's that.

I suppose it'd be worth mentioning, but I'm running Sunoco 260 GT, a CARB legal 100 octane race fuel.
 
Stumble at idle is a misfire and that sounds like your problem. There isn't a definitive test for the MAF or 02 that I can think of. The O2 oscillates between 0.1 to 0.9 V as the mixture is adjusted by the ECM. You can check if it's pegged at either extreme, but it's tough to interpret a raw voltage reading.

MAFs you just gotta swap out. I remember lostartof finding out his mixtures would change based on which MAF was in the car at the time.

Injectors can stick and leak raw fuel in, or be restricted. The computer will compensate for a restricted injector by fattening up the mixture on ALL 4 injectors.

The cat... a fresh one may be a bandaid, and you MAY pass, but if you are misfiring, a cat ain't gonna do it.
 
Stumble at idle is a misfire and that sounds like your problem. There isn't a definitive test for the MAF or 02 that I can think of. The O2 oscillates between 0.1 to 0.9 V as the mixture is adjusted by the ECM. You can check if it's pegged at either extreme, but it's tough to interpret a raw voltage reading.

MAFs you just gotta swap out. I remember lostartof finding out his mixtures would change based on which MAF was in the car at the time.

Injectors can stick and leak raw fuel in, or be restricted. The computer will compensate for a restricted injector by fattening up the mixture on ALL 4 injectors.

The cat... a fresh one may be a bandaid, and you MAY pass, but if you are misfiring, a cat ain't gonna do it.
I can try for a video tomorrow, but you're pointing me fairly confidently towards a MAF....I MIGHT have a spare (don't think so)

I don't want to fire the parts cannon, so I can test O2 sensor in the morning, and post results.

EDIT: I found my spare MAF from the JVAB Collection!
 
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O2 sensors can get "lazy" but you aren't gonna see it on a meter. I rely on modern OBD to determine that

I would put a fresh high quality O2 sensor on there for MPG reasons anyways. It's something I think I have overlooked on these cars in the past.

I work on "modern" cars where the O2 oscillates as expected, but still may throw a code against it.

Ol' LH2.4 isn't gonna throw an O2 code unless the heater circuit is open, or there's something really wrong.
 
O2 sensors can get "lazy" but you aren't gonna see it on a meter. I rely on modern OBD to determine that

I would put a fresh high quality O2 sensor on there for MPG reasons anyways. It's something I think I have overlooked on these cars in the past.

I work on "modern" cars where the O2 oscillates as expected, but still may throw a code against it.

Ol' LH2.4 isn't gonna throw an O2 code unless the heater circuit is open, or there's something really wrong.
I've gotten very complacent with my OBD2 cars


It would be awesome if the MAF solves the idle "misfire" Thats been about the only "red-flag" for me since getting it back together.
 
Do you have your valve lash all dialed in?

You may wanna try different injectors.
PO said he adjusted lash. I've never even tried to measure lash on a vehicle myself.

Been on the fence about ordering a rebuilt set anyway. (injectors)
 
I figured you may have set the lash since you just had the engine out...

I'd go with fresh 02 sensor, fresh clean injectors, try again.

I experimented on a 1986 740 and it took injectors to finally get it thru smog. My old trick of putting a fresh cat didn't work.
 
Cool, I'll order a o2 sensor....probably the injectors.

Will test the spare AMM tomorrow and HOPE my stumble at idle disappears.
 
First off, look at your O2. You've got O2 coming out the tailpipe along with HC, so the cat isn't using the available O2 to burn the HC. Cold cat could do it, or a tired cat. That's my first clue.
O2's, do NOT go with Bosch, they have tanked in quality. NGK/NTK have been really solid with more accurate readings and faster response times. Given the miles, I'd go that before a MAF to be honest. You can also clean the MAF if you use the proper cleaner.
Mine has always had a slight random misfire at idle, for YEARS, nothing has ever changed it, but I've never had an issue passing smog either.
 
First off, look at your O2. You've got O2 coming out the tailpipe along with HC, so the cat isn't using the available O2 to burn the HC. Cold cat could do it, or a tired cat. That's my first clue.
O2's, do NOT go with Bosch, they have tanked in quality. NGK/NTK have been really solid with more accurate readings and faster response times. Given the miles, I'd go that before a MAF to be honest. You can also clean the MAF if you use the proper cleaner.
Mine has always had a slight random misfire at idle, for YEARS, nothing has ever changed it, but I've never had an issue passing smog either.
What do you clean it with?

I've got access to NTK

Cat wasn't cold...130 miles of hooning to get to shop and they put it right on the dyno
 
I will be very interested to see if you pass with a fresh O2 and injectors...

I had to study this stuff for all the ASE tests years ago, but it's not stuff I deal with commonly. Many/most? shops don't even have the dyno anymore.

Good luck. I have a 1991 744 that failed for evap, and my 1991 745 is blowing smoke. I live close to work, use the GFs car, and I'm paying registration on cars I'm not driving....
 
There are still some places in CA with NO smog test, I think...

I'm guessing in rural they have phased out the dyno....as they phase in this test....idk
 
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