: 94 STS High HC at Idle

04-12-08, 11:15 PM
94 STS 168K miles just failed CA Smog for High HC at Idle - gross polluter!
Reading is 1486 @685 rpm, only 11 at 2462 rpm
CO is in spec

Engine seems to idle a little rough. If you touch the gas pedal, it's hard to sustain a slightly higher idle (say ~ 1000) But if you enter the diag test mode for turning off each injector, it idles at fixed ~1100 rpm - still seems a little rough there but none of the cylinders seems any weaker than the others when the fuel is cut to each injector.

A couple of tests ago failed similarly - did almost everything on that go-around (FPR, cleaned EGR, cleaned plenum, new Injector seals) finally cured with new plug wires - actually replacement AC Delco wires were just as bad, could see major arcing visibly. I put in a set of Magnecore 8mm leads and that fixed it on that occasion.

This time there is no sign of any arcing, even when misted with water from a spray bottle.

With the emission analyzer in the tail pipe, in manual mode, we watched the HC as the engine rpm increased - almost immediately it would recover down to single digits & remain there for a few seconds before it would creep back up again.

Any ideas what these symptoms point to? Definitely seems like mis-fire condition .....

FPR maybe?

p.s. No relevant codes indicated

04-12-08, 11:42 PM
FPR is very easy to diagnose. Just pull the vacuum hose at idle and look for fuel leakage.

04-12-08, 11:48 PM
FPR is very easy to diagnose. Just pull the vacuum hose at idle and look for fuel leakage.
Thanks for quick reply - it's 94 N*, the FPR is inside the plenum I believe so no vacuum hose.
I'm going to pull the cover & check it tomorrow - meanwhile, looking for thoughts on what the specific symptoms reflect, especially with the HC levels recovering at even slightly higher rpm
Is this indicative of FPR or ???

04-13-08, 05:33 AM
Also, Clean the throttle body and do a WOT (Wide Open Throttle) procedure to clean out carbon build up. And yes the FPR is in the plenum with no vacuum hose. Don't start the engine with the plenum off! Just turn the key to the on position to pressurize the fuel system and look at the nipple on the FPR to check it.

04-13-08, 09:11 AM
Since yours is a '94 and inside the manifold, you are correct, no hose. In your case, just go key on to pressurize the fuel system, but DO NOT start the engine.

04-14-08, 06:56 PM
FPR is bone-dry - no issues. I'm pretty sure I replaced that in the last slug-fest - was worth checking but not to be.

Biggest problem is diagnosing without an analyzer - motor runs pretty good except for a slight uneven idle.
All the cylinders behave similarly when doing the Over-ride program 4 injector kill tests - none seems to affect it any worse than the others & even then all only have a very subtle effect.

EGR is good / clean

Any other suggestions appreciated.

04-14-08, 08:11 PM
Vacuum leak?

04-14-08, 08:20 PM
Yeah - what Ranger said - I was thinking that - it may seem counter-intuitive, but a vacuum leak can cause high HC due to a lean miss at idle. The un-metered air would become insignificant at higher engine speeds, so it would run fine.

04-14-08, 08:37 PM
Hmm - Interesting thought - I would have suspected high idle though, or surging?
I'm not seeing any obvious areas with hose problems etc.
The EGR was cleaned (wasn't bad at all) - the ISC was definitely replaced last time, all looking good on those fronts.
Looking at the plugs # 8 is a little funky - the others look like poster shots - perfect light taupe colour.
#8 is much darker, deposits around the center electrode and insulator.

Bad Coil maybe or ????
I looked at # 3 plug as well though (shared coil with 8) but it looks squeeky clean however.


04-17-08, 02:28 PM
hey guys first post for me on this site but have to say great site i have the exact same problem down to the #8 plug on my 1994 sts with 74000 km on it i have done the plugs and i guess the fpr check is next i heard about cleaning the mouse holes or some plates?> is this all located inside of the sealed plenum or under it? and more ideas would be greatly appreciated thanks in advance

04-17-08, 04:28 PM
Yes, all located inside the manifold.

Phil Errett
04-18-08, 09:14 AM
I just experienced the same issue when my 99 STS failed its e-test the first week in March.

Problem explained to me as fouled plugs - just failed on idle @ 54 - plugs changed - 8

Mechanic suggested common problem with N* - cylinders out of round and when under WOT rear window look show a tone of shit flying out.

Love the car - hate this problem - uses 1 litre every 1500 kms.

04-18-08, 01:38 PM
Phil, Go to the top of this page, in the black area, and click on Technical Archive, lower left in the bar. Read everything, but particularly "Northstar Oil Consumption". Northstar cylinders are not commonly reported in here as being out of round. Maybe it's a trade secret......

Never fill your oil to more than halfway up the dipstick hashmark. If you overfill to the top of the hashmark the engine will rapidly burn off the excess 3/4 quart/ 1/2 L.