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93 Posts
Discussion Starter #1
I have a '91 S-10 4.3 L. I'm live in California and it won't pass the emissions test. The only category it fails is the "NO - 15 mph" test.

Before I throw money at parts I don't need. What is the cause for the truck to fail the 15 mph NO test?

O2 sensor, cat. converter, EGR. could all be a cause, but if try to test each one the best I could.

What about the EGR vacuum soleniod?

Any advice would be great! Thanks Kevin

10,898 Posts
Welcome to the site..Can you post the VIR with the readings ? I live in Ca
as well as might be able to help. If you have the orig o2 sensor in there
as well as the orig cat they will probably need to be replaced. If you
decide to get a new o2 sensor try and get one from Delphi or AC/Delco.

Here's yours.

Here is a site that has good prices on Delphi o2 sensors. As far as a
cat you have a few options. I run a direct fit Catco, but Magnaflow also
makes very good cats. I also had a problem with mine. I serviced the
EGR passage and the valve itself bu pouring some carb cleaner into
the EGR passage and using a special EGR flex bit I bought to reach
WAY down and clean it up. Once I got most of the way down and let
the carb cleaner sit for a couple hrs, I covered up everything as well
as I could and started the engine with the ERG valve off. You should
of seen the crap that came flying out from revving it a little. It really
cleaned out the passage.

I then installed a new gasket and the old EGR valve, did a fresh oil
change and made sure everything was hooked and went and smogged
it. Passed ok. This is at least a start. I hope the guy that tested it
had the fan sitting right in front of the rad while doing the test to help
keep it cool. The smog tech is suppose to place a large fan right
in front of the ride when the temp is over 73* when doing both the
15 and 25 test.

Seeing what the readings were will help a lot. So if you can post them.

93 Posts
Discussion Starter #3
Thanks for the info! I've enclose my emission report (hope you can make it out) . The cat. and o2 sensor and the EGR valve have been replaced, but a handful of years ago.

What about the EGR soleoid? Does that have anything to do with it?

%CO2 %O2 HC (PPM) CO (%) NO (PPM)
15 mph 2278 14.7 0.4 129 316 15 0.80 2.10 0.00 1037 1831 2752 GP

25 mph 2197 14.6 0.3 109 266 25 1.01 2.31 0.23 897 1631 678 Pass

So in your best judgement, what would you replace? Thanks for your time! Kevin

10,898 Posts
Can you break that up some ? Maybe in a quasi column like what's on the VIR ?
That list you have is very difficult to read. it looks like you failed for high NOx
and HC's. In fact it looks like you are gross polluter. That's not going to be fun.
I ALWAYS ask for a pre-test so as to avoid this. From now on you'll be going
to test only stations for your smog test....Trust me.

Yea the solenoid might be the problem, but I would make sure the timing
is set at "0" in base mode, that you do a major recarbonization of the
engine using GM Top End Cleaner or Seafoam, pour good injector cleaner
in the tank or better yet have the injectors professionally cleaned, make
sure you have a fresh fuel filter installed and that you have enough fuel
pressure going into the injectors and service the EGR system including
the passage. You're failing at 15 mph when the dyno puts the most load
on the engine. That causes hi temps in the cylinders. Anything over 2500*
in the cylinders will cause hi NOx and a possible misfire. It appears that
the EGR system isn't doing it's job real well and that there is a misfire'
in there somewhere since it looks from what I decipher from your listing
high HC's. Did you GP on the HC's part of the readout as well as the NOx ?

Another thing is you ALWAYS want to change the oil on the day of the test.
You always want to smog it in the mornings when it's still cool out. You
want to let the ride idle while you submit your paperwork to the smog tech
in order to keep the cat hot. Try and go for a good road trip right before you
test in order to lite the cat off. Older s-10 rides have a real problem keeping
the cat lit. You will also want to make sure you NO VACUUM LEAKS. They
will cause a misfire and high NOx as well. You can find vacuum leaks using
a small stream of propane to search for when the truck is idling.

Make sure the PC Valve is working right and that the little black elbow that
slips onto the front base of the TBI body from the PC Valve's tube is NOT
SPLIT. They are infamous for splitting underneath where you can see it.
It will cause a major vacuum leak and you'll end up with high HC's and
high NOx.

There are tests you can do on the EGR system that will help you determine
if it is working properly. You can do a quick one right at the house. Take
the air cleaner off so you can access the ERG valve. With the engine
at idle reach up under the valve and push the diaphragm open a bit.
If the engine stumbles and wants to die, and returns to normal running
once you let the diaphragm close again, you can figure the valve may
be working ok. I'd STILL take it apart and soak the passage with carb
cleaner and then start it up with the valve off to blow that nasty a$$
carbon buildup out of there..

Have you done a tuneup recently ? I always try to do all the maintenance
before the smog test, That way you've done as much as you can with
out diagnostic tools. HTH's

93 Posts
Discussion Starter #5
%CO2 %O2 HC (PPM) CO (%)
15 mph 2378 14.7 0.4 129 316 15 0.80 2.10 0.00

25 mph 2197 14.6 0.3 109 266 25 1.01 2.31 0.23

15 mph 1037 1831 2752 GP
25 mph 897 1631 678 Pass

I did pre test it the first time so I wouldn't waste money. And it was a "TEST ONLY" station.

Do these guys know what there what there doing? They also said it was out of time 5% degrees, but I don't think they disconnected the wire under the dash before they timed it. I timed I when I got home the right way and it was fine. Could that cause a fail?

I've put new plugs , wires and rotor on 10k ago. Should they be done again?

I also put a infrared temp gun before and after the cat. and there was only a difference of 40-50 degrees.

5,065 Posts
EGR reduces the NOx. The solenoid operates by the computer so you only have the egr working at part throttle and cruise. It would idle poor if the egr was working at idle. The EGR is lower combustion chamber temps to lower the NOx. Maybe your timing being off is causeing the temps to raise also. Test only guys need to know what they are doing or they cant stay a test only. They hook a lead up to the plug wire that hooks to the machine. If it tells them its off then its off. What did you time it to? Check your EGR. I can scan and email you a flow chart that might help you to test your solenoid. Or I will let you in on a secret. BG's had a CD that covers all types of good stuff and is really cool . He might be able to send you a good copy of a test procedure.

10,898 Posts
Well if the cat wasn't lit off, you'd have that low a reading. But if the WAS lit
off, you should have a min of 100* hotter coming out of the cat than going in.

As Mike states the timing being 5* off IS a big deal. What I'm concerned with
is the fact that the tech didn't unclip the ECM timing interrupt wire under the
pass side carpet. If they didn't do this, then they did an incorrect test and
you can file a complaint to BAR. When you got it home and checked timing
with the wire disconnected, what reading did find on the timing light ? It
should be "0" degrees.

I looked at the readings and I believe I'm seeing 15PPM's for HC's and
0.00% for CO. Is that right ? If so, it looks like it may be running really lean
at the 15 test. No CO could cause a lean misfire=heat and possibly high
NOx failure.

Here is a tree probably like the one Mike offered. At least you start testing

93 Posts
Discussion Starter #8
Mike, Can you send me the flow chart to help me check the EGR solenoid? The would be great! Thanks! Kerwin

93 Posts
Discussion Starter #9
BG, The wire was connected, when I got home, I then disconnected it and timed it myself and I was 0 degrees.

Yes, your reading the numbers correct. What should I do about a lean mixture at 15 mph?

Thanks. Kevin

10,898 Posts
Hmm. I don't know, but if you didn't see them disconnect the wire and do a
base timing functional test, they did it wrong. Anway, as far as getting MORE
fuel, I would suggest a brand new fuel filter to maybe help with flow. That
should also be done right before a test. Other than that you might have to
increase the fuel pressure and if your not familiar with the way the 220
TBI works, you may have probs. I'll give you a link and see if you get
how to modify the fuel pressure reg, but this IS a mod and you have to be
careful with what you do. Another factor may be in the fuel trim and that's
ECM related.

So first off I would check to see what fuel pressure is. This is hard on a
TBI as GM tells you to remove the filter and install a "test block" where
the filter was and then hook up a gage. I found this to MUCH easier and
not bad at all on price. Once you install the fitting between the TBI
and the inlet fuel line you can test fuel pressure any time you want. >


You can probably find this at your local Auto Zone. You just want to make sure
it has that brass fitting that has the schrader valve sticking out of the side. This
is made for GM TBI's and really makes testing fuel pressure easy. You
want about 11# of fuel pressure. GM sets them up between 9 to 12#. I
found mine runs best at 13.25.

It looks to me to be running too lean. Have you ever changed out the pump
or the strainer that is on the end of the pump inside the tank ? If not there's
every chance the strainer is stopped up some from all the years of collecting
fuel from the tank via the pump. The pump itself might be weak. I had to
change mine out and in doing so just went with a Walbro. I think you really
need to know what fuel pressure is. Here is a link showing how to modify
the stock reg. This is up to you. I'm not saying to modify ANY component
that may effect emissions as that's against the law. I have found though
for those rides that are used "off-road" this is a great mod for upping fuel
pressure for TBI engines. >


I posted a tree to help with the EGR in post 7 at the bottom. You may end
having to take the ride to a shop that does repairs on gross polluters and
that also certifies them. You might want to contact BAR and find shops
that can do this..>


Just remember there aren't any cost limits on repairing gross polluters
unless Ca has changed their policy since my last smog license. That
was in 2000 so things may have been redone.The people at the BAR
with tell you all the "good" news. Once a GP, the nightmare begins
every two years..:(

93 Posts
Discussion Starter #11
Is there a test for the EGR solenoid?

If have about 10 K on the plugs, wires and rotor, should I change them?


5,065 Posts
Scroll up from here and check at the bottom of post #7. Bg's left a link to a flow chart.
And 10,000 is not enough to have to change that stuff again.

10,898 Posts
I still think finding out what fuel pressure is at idle and at say 2000 is
important. That's 4 years on the pump and strainer, that's not bad. You
didn't mention anything about the inline fuel filter. You're getting good CO2
out of the exhaust with a just a lil bit of o2 so at least the cat seems to be
breaking down the spent exhaust. HC's are pretty low, the problem is NOx
and here is what causes high NOx. Too far advanced timing, a lot of carbon
build up on top of the pistons, insufficient cooling, vacuum leaks, a lean
condition, a faulty ERG system, a misfire to an extent, clogged or leaking
injectors, a biased o2 sensor or slow acting o2 sensor, and or one or more
of the sensors sending incorrect voltage to the ECM. For example a MAP
sensor that is not seeing enough vacuum due to a leak or split line from
the TBI will not send the correct voltage to the ECM so it can pulse the
injectors on longer. You mentioned that the tech stated the timing
was 5* too far. I'm guessing he meant too far advanced. When you
look at the VIR did he fail you on the function part of the test for
timing ? It will be listed on top of the gas reading columns.

I think and I'm guessing, that when you went to test, you didn't drive the
ride that long to get the cat lit off, and when you went to the smog shop,
you shut off the engine and the cat cooled down. The smog tech took
all the info, had you sign the repair order and then went and got your
ride and tested it with cat cooled down some, or the truck sat for while
not running while the smog tech finished a ride he was getting ready
to test and again the cat cooled down. With these older rides just about
everything has to be working good and the cat hot. That's why I always
leave mine running at idle or sit in it doing a semi high idle keeping the
cat lit, so when it's pulled on the dyno, it has a better chance of going
thru. I have a 92 with a 4.3 TBI.

From where I'm at, I think at least get a new o2 sensor and Seafoam the h*ll
out of the engine to break up the carbon. You can get Seafoam at Napa.
I prefer GM Top End Cleaner, but I've only found it at GM dealers. I'd clean
out the injectors either with Seafoam or if I had the money, use 3M's injector
cleaning kit to clean out the injectors, change out the fuel filter, change out
the oil which you MUST do after de-carbonizing the engine, go for a good
ride to blow all the rest of the carbon out the tailpipe and find a Gold Shield
Station close to you and talk to them about the situation. Of course you can
test the EGR system out yourself and that's one other thing I would service.

Other than that, it's all I can come up with. Getting more fuel to the injectors
or more fuel pressure I've linked up in one of the other posts. Mike has
offered to send you a scan of the tree for the EGR. I'm sure it would be of
better quality than my link above. HTH's You can always PM with any
questions so as to save bandwidth.

93 Posts
Discussion Starter #15
Thanks BG so much for all your time on this problem!

I change the fuel fliter 40 K ago.

Yes, I said it failed the timing test.

I drove the truck 12 miles the get it testing and it sit 30 mins. cooling down before they tested it.

Thanks again

5,065 Posts
40k on a fuel filter is a lot. I try to change mine at least every other oil change.
Did the shop let the truck warm up before testing or just fire it up and put it on the machine? They should have started it and had it running as they enter info on the machine and do the visual inspection. The last time I got my car smogged I drove there and was the only one getting a test. The tech took my keys and paper work out to the car and started the car, filled in the milage, plate..etc. He then pulled the front into the door way and opened the hood and did the visual all while the car was still running. Entered that info into the machine then pulled it onto the rollers and started the test. My car sat for about 2 minutes of cooling down and 5 minutes of idleing before he began the test.

93 Posts
Discussion Starter #17
Mad Mike, I don't know. They said it would take 1/2 hour to get to it. So I left. BG said in this thread the the cat. has to be HOT. So what idling it for a couple of mins. by the tech get it hot enough?

5,065 Posts
Mad Mike, I don't know. They said it would take 1/2 hour to get to it. So I left. BG said in this thread the the cat. has to be HOT. So what idling it for a couple of mins. by the tech get it hot enough?[/quote]
I had a car fail because it sat for over a half hour and the idiot did not precondition the cat long enough before putting it on the machine. I took it to another shop and they did the warm up and it passed. Go figure! So to answer your question, most likely not.

10,898 Posts
Yea as Mike says if the cat isn't hot it won't fly. What it states in the smog
procedure book is that the upper rad hose must be hot before testing. This
tells the tech that the t-stat opened up. If the ride just sat and then the tech
fired it up and then just drove in on the dyno and did the test after putting
the info in, then he did it wrong. I think you have a good chance of filing
a complaint and taking it to a referee for another test, if you believe the
tech did it wrong. You can contact BAR and ask them..>

Ya have nothing to lose really. It's already a GP and maybe the rep at
BAR can help you..Just don't trust them too much. >


93 Posts
Discussion Starter #20
BG, Can you explain exactly how to use Seafoam? how much, where to put i.t (gas, which vacuum line, etc.) Do I leave the Seafoam in the gas tank for the smog test?
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