Fuel mix is rich - I need advice! (Page 1/2)
decrawford1 DEC 08, 07:39 PM
I'm a new Fiero GT V6 owner and need experienced advice. The engine runs steady with plenty of acceleration and idles at 1,500 RPM. No SES warning light. I should be a happy camper, right?

Here's the problem: the fuel mix is rich, evidenced by blackened spark plugs, oil that smells like gas, and rich-smelling exhaust. So here's what I've replaced:
-- MAP sensor
-- Throttle position sensor
-- MAT sensor
-- Coolant temp sensor
-- O2 sensor (existing one wasn't plugged in)
-- Vacuum lines
-- Fuel pump and fuel filter
-- EGR valve
-- Spark plug wires and spark plugs
-- Oil and oil filter

But no change. Except now it idles at 2,000 RPM unless I unplug the O2 sensor, then the idle returns to 1,500 RPM.

I'm perplexed and need wise counsel.
Skybax DEC 08, 08:02 PM
Welcome and congrats!

You said no SES light, but did you check to see if the bulb works or to see if somebody removed or taped over it? Turn ignition on and see if it lights up and check for stored codes.

Most common cause on V6 Fiero is cracked EGR tube causing vacuum leak, sucking air into intake, causing computer to compensate with richer mixture and higher idle. (normal idle speed is around 925 at full operating temp when everything is working correctly) The EGR tube is usually cracked near where it attaches to the intake, look to see if you can see black soot around it, peal back the insulation to look further down the pipe. If yours is cracked you can buy a new tube here... http://rodneydickman.com/pr....php?products_id=308

Note: You don't have to remove the intake to replace EGR tube, just remove distributor cap and rotor and you have enough room, that is the easy part. The bigger problem is where it attaches at the EGR valve, which is an intermediate plate and the nuts often break off the studs when you try to remove them. If that happens you have to remove the EGR and intermediate plate, heat up the area around the broken studs on the intermediate plate with acetylene torch (get it glowing red) and carefully remove the studs with needle nose vise grips. Once the broken studs are removed you can attach the new EGR pipe to it using new bolts with same metric threads.

Also, not mentioned on your list is removal and cleaning of IAC valve (idle air control) and it screws into the throttle body. Be very careful removing and reinstalling, tricky, it can break easily. All you have to do is clean the nipple and the seat in the passage of the throttle body with some carb cleaner on a rag. (check EGR tube first)

Lastly, the other thing you want to check is to see if anyone messed with the throttle plate stop screw on top of throttle body above spring. (here is photo of it) Its set and sealed from the factory, but some people remove the cap and mess with it thinking its the idle screw. If it still has the cap seal you're good, if the cap seal is missing I suggest re-calibrating it ONLY after you fix other problems. Here is the procedure I wrote in 2005 (scroll halfway down) and a few other good tips... https://www.fiero.nl/forum/...060206-2-060970.html

[This message has been edited by Skybax (edited 12-08-2021).]

Gall757 DEC 08, 09:06 PM
Fieros are just smart enough to need a little driving time to adjust to all those new parts. Some of your issues might go away after a 15 minute drive.
decrawford1 DEC 08, 09:08 PM
Skybax - Yes, the SES lights up when I turn the key, and I received a code on another issue a couple of weeks ago, so it appears to be working.

I didn't know about the EGR tube so I'll go ahead and order a new one from Rodney. I have not removed and cleaned the IAC valve so I'll do that this weekend. Thanks for warning me about being careful removing and reinstalling it. I'll also check on the throttle plate stop screw.
decrawford1 DEC 08, 09:20 PM
Gall757 - If I could be so lucky! I driven it for several hours to no avail. It's a great car.
OldGuyinaGT DEC 08, 11:32 PM
Previous posts are correct - most likely you have a large vacuum leak, and the most likely source of one is the EGR tube. I had this when my '88 GT was new to me. It took me a while to troubleshoot it, and the rich exhaust killed a new cat in the meantime (had to replace it again after I fixed everything else). Never did throw a code. While there are other possible sources of vacuum leaks, most are smaller and so will have a smaller effect on idle RPM, etc. Rodney's EGR tube is a quality part; I even used his modified discounted one and it went in easily and is just fine after about three years/25k miles. Like others have also said, the problem could also be related to the IAC.

Something that would be useful here is an old-fashioned vacuum gauge. It'll tell you if the problem is in fact a vacuum leak, and can be used to verify repairs of vacuum leaks.

I would also check the ignition timing, just to cover all the bases. If you have good power, timing probably isn't very far off, but it can contribute to a rich condition and incorrect idle speed. Check the timing after correcting any vacuum leaks; timing will be difficult to set accurately if the idle speed is incorrect.
fierofool DEC 09, 08:46 AM
Two cautions. EGR tube and IAC.

There is a small orifice washer that fits into the end of the tube where it bolts underneath the intake manifold. Be sure it's in place. It's to prevent a runaway idle in the event the tube gets a major break.

The other warning is the IAC. It shouldn't need readjustment just to remove it and clean it. Don't spray it with any type carb cleaner or solvent. It can damage it. Just apply some cleaner to a wipe and clean the tip. You cn spray cleaner into the IAC port then blow it out with air. I used a rifle bore brush to lightly clean the port in the throttle body. Also, just to be sure the IAC adjustment didn't change while cleaning or if you replace it, the measurement from the shoulder of the IAC where the gasket sits, to the top of the pintle should be not more than 1 1/8 inch. It can be less, but not more. The gasket should removed to make the measurement. The ECM will need to readjust the IAC so idle may be erratic until driven.

There are two styles IACs. One has a smooth pintle shaft. This requires retracting the spring from the pintle head and screwing the pintle in or out. The other has slots on the sides of the pintle shaft. This requires using pressure while either pulling or pulling the pintle head in or out as you rock it side to side. Turning it will break the tabs that fit into the slots on the shaft.

When you replaced the throttle position sensor, did you probe the circuits to see if you had 0.5 volts at idle? There's no guarantee that the sensor is properly set for your car.

AND, I've owned a few Fieros. All V6's except 1, but all had dark exhausts and all the V6's usually idled around 2000-1100 when warmed up. All manual transmission cars.
decrawford1 DEC 09, 10:59 AM
OldGuyinaGT - Sorry to hear about your cat! ;-) I ordered a new EGR tube from Rodney last night and he shipped it this morning. Good idea on the vacuum gauge - I'll pick one up and learn how to use it. I'll find someone to check the timing because that's above my pay grade! Thanks for your help.

[This message has been edited by decrawford1 (edited 12-09-2021).]

decrawford1 DEC 09, 11:05 AM

quote
Originally posted by fierofool:

Two cautions. EGR tube and IAC.

There is a small orifice washer that fits into the end of the tube where it bolts underneath the intake manifold. Be sure it's in place. It's to prevent a runaway idle in the event the tube gets a major break.

The other warning is the IAC. It shouldn't need readjustment just to remove it and clean it. Don't spray it with any type carb cleaner or solvent. It can damage it. Just apply some cleaner to a wipe and clean the tip. You cn spray cleaner into the IAC port then blow it out with air. I used a rifle bore brush to lightly clean the port in the throttle body. Also, just to be sure the IAC adjustment didn't change while cleaning or if you replace it, the measurement from the shoulder of the IAC where the gasket sits, to the top of the pintle should be not more than 1 1/8 inch. It can be less, but not more. The gasket should removed to make the measurement. The ECM will need to readjust the IAC so idle may be erratic until driven.

There are two styles IACs. One has a smooth pintle shaft. This requires retracting the spring from the pintle head and screwing the pintle in or out. The other has slots on the sides of the pintle shaft. This requires using pressure while either pulling or pulling the pintle head in or out as you rock it side to side. Turning it will break the tabs that fit into the slots on the shaft.

When you replaced the throttle position sensor, did you probe the circuits to see if you had 0.5 volts at idle? There's no guarantee that the sensor is properly set for your car.

AND, I've owned a few Fieros. All V6's except 1, but all had dark exhausts and all the V6's usually idled around 2000-1100 when warmed up. All manual transmission cars.



Your cautions are much appreciated! I'm definitely outside my comfort zone.

decrawford1 DEC 27, 10:37 AM
Update - better, but still perplexed!

I received the new EGR tube from Rodney and installed it, and also installed a new IAC valve. I checked the throttle plate stop screw and it does not appear that anyone has messed with it. I did discover that the vacuum line to the fuel pressure regulator was loose, so I plugged it back in.

Hurray - the engine is no longer running rich! I drove it for over an hour, checked the spark plugs and they're spotless. However, the idle is still high - about 1,900 RPM.

Here is my question - I've done some more reading and have learned about the IAC air tube (or cold start valve air tube per the image below). If it's leaking air, could that cause my idle to be high?