Strange idle - stock '95 GT

SIIaCanuck · Jan 20, 2016

This morning on the way to work I ran into an odd idle issue.I pulled off the highway after cruising for an hour+ and arrived at some lights. As I downshifted (5 spd) I noticed that the rpm wasn't dropping between shifts but hanging up around 2000rpm.When I stopped at the lights, it sat at 2000rpm for a few seconds then slowly decayed to 1600. I blipped the throttle and it dropped back to about 1400 and then slowly settled to about 1250 by the time the lights changed (couple of minutes, it's a busy overpass with a long light delay).It continued to repeat at each stop/corner to work (another 4 stops). Once in the parking lot, I let it settle to 1400ish then cut the engine and immediately restarted. It started completely normally and settled to a normal idle with no issues.Any thoughts on what might be happening? Am I looking at something failing (sensor)?It's a stock (engine) '95 GT with about 175,000km on the clock (110,000mi). I'm running a 3.9 rear end with a stock T-5 and a DM SpeedCal to correct the speedo. I cruise at about 2350rpm on the highway at around 110 kph (70mph).Stew

SIIaCanuck · Jan 20, 2016

Please ignore the long paragraph, my browser is ignoring paragraph spacing for some reason.

Bauman_Octane · Jan 20, 2016

SIIaCanuck said:
Please ignore the long paragraph, my browser is ignoring paragraph spacing for some reason.

so just throwing some random tips and guesses out there. i would check all the vaccum lines from the intake manifold to start.probably not your problem but weirder things have happened. someone can correct me if i am wrong but having the idle take a while to come back down could be signs of the IAC (idle air control) getting old. i would say fuel pressure regulator but your car would be doing the opposite. other ports on the throttle body and intake clogged? throttle cable? all places to start.

CC'S95GT · Jan 20, 2016

Mine was doing the exact same thing but only in the winter months. When it was warm it wouldn't hang at all.
I believe it's the TPS. The TPS is a rheostat and has probably gotten a dead spot in the 20 years of use. You can test it if you have a multimeter.
An analog meter would be better for the test but a digital meter will work too.
You can test it still connected by back probing the connector. Just pick any 2 or the 3 wires.
Set the meter on VDC.
Turn on ignition (no need to start it).
The voltage with the throttle closed should be aprox .5vdc.
Very slowly turn the throttle and watch the meter.
The voltage should increase as the throttle increases.
If the voltage drops anytime during the cycle repeat the test and make sure it drops in the same spot everytime.
If you have confirmed a dead spot replace the TPS.

evilcw311 · Jan 20, 2016

It's either a vacuum leak or bad iac. Mine did the same twice and it was vaccum once and iac other

This message courtesy of crapatalk!

g36 monkey · Jan 20, 2016

Do a vacuum check because it's likely vacuum related. Get a cigar, remove the vacuum hose from the brake booster, exhale smoke into the hose, watch for leaks out of the motor. If that doesn't yield results, then look into IAC and TPS. I figure it's a great diagnostic tool for about $1.

Bauman_Octane · Jan 20, 2016

g36 monkey said:
Do a vacuum check because it's likely vacuum related. Get a cigar, remove the vacuum hose from the brake booster, exhale smoke into the hose, watch for leaks out of the motor. If that doesn't yield results, then look into IAC and TPS. I figure it's a great diagnostic tool for about $1.

haha i can think of better smoke related products than a cigar. wink wink..but actually thats a pretty sweet tip.

evilcw311 · Jan 20, 2016

g36 monkey said:
Do a vacuum check because it's likely vacuum related. Get a cigar, remove the vacuum hose from the brake booster, exhale smoke into the hose, watch for leaks out of the motor. If that doesn't yield results, then look into IAC and TPS. I figure it's a great diagnostic tool for about $1.

What kind of cheap arse cigars you buyin for a dollar??!? If I'm gonna buy and burn a fatty I want to enjoy it and the ones I enjoy range from about $8-$25.

This message courtesy of crapatalk!

BranSVT · Jan 21, 2016

Trying to figure this issue out on my 94 cobra. Whenever I shift (or clutch is pressed in while driving), the RPMs increase +300/+400 instead of decreasing like normal.. coming to a stop, it stays at 1200 -1400 till I'm under 5mph, then the idle goes down to normal 900-950. Haven't tried any of these tests yet.

SIIaCanuck · Jan 22, 2016

I'm not sure I want to be anywhere near a $1.00 cigar!

The problem hasn't repeated since but I'll run through a few of those checks. The TPS idea seems to make sense because it has a slight cough, when not fully warmed up, at a throttle position just off idle. It only happens if I'm rolling around a slow corner in third and then feed in gentle power so I'm hitting the right throttle position. Above and below that position it runs normally.

Thanks for all the ideas and advice. If anyone knows where I can get a decent $1.00 cigar, let me know. I reckon they're as rare as unicorns but I could be wrong.

evilcw311 · Jan 22, 2016

SIIaCanuck said:
I'm not sure I want to be anywhere near a $1.00 cigar!

The problem hasn't repeated since but I'll run through a few of those checks. The TPS idea seems to make sense because it has a slight cough, when not fully warmed up, at a throttle position just off idle. It only happens if I'm rolling around a slow corner in third and then feed in gentle power so I'm hitting the right throttle position. Above and below that position it runs normally.

Thanks for all the ideas and advice. If anyone knows where I can get a decent $1.00 cigar, let me know. I reckon they're as rare as unicorns but I could be wrong.

Decent, $1.00, and cigar.............

3 things that don't go together!!!!

This message courtesy of crapatalk!

96laseredcobra · Jan 22, 2016

I vote IAC valve. Sounds all to familiar. As both my rides did this at the same time.

sent while thinking of mods for my 4.6s using tapatalk

ranger56528 · Jan 22, 2016

evilcw311 said:
Decent, $1.00, and cigar.............

3 things that don't go together!!!!

This message courtesy of crapatalk!

Exactly what I was thinking,wish I could bring a box home from Canada but I guess I'll have to settle with my Dunhill and Fuente's.

evilcw311 · Jan 22, 2016

ranger56528 said:
Exactly what I was thinking,wish I could bring a box home from Canada but I guess I'll have to settle with my Dunhill and Fuente's.

Acid by Drew estates.

I smoke from the blue labels. If I want a quick burner I get the blondies and if I want a fatty I get the Kuba Kuba.

My all time favorite is their Ambrosia Mother Earth.

This message courtesy of crapatalk!

ranger56528 · Jan 22, 2016

I like the Privada #1 or the Condados for the most part but it seems the Privada can be a hard one to find sometimes but ya a good Churchill will do,Dominican makes a good cigar.

BranSVT · Jan 22, 2016

SN95 forums, from cars to cigars.. Lol..

ranger56528 · Jan 23, 2016

Back to ops issue sorry got off track hahahatrack cars cigars.

Sent from my iPad using Tapatalk

g36 monkey · Jan 23, 2016

I still recommend the vacuum check before you start buying replacement parts. 99% of the time the idle issue is a vacuum problem.

SIIaCanuck · May 3, 2016

Well, cigars or no cigars, it was a dead spot in the TPS. Threw both a 122 and 124 in the memory so the TPS was giving some poor signal in the idle-low throttle range.

Replaced with a new TPS and the hanging idle disappeared . . . for about three weeks.

Then, as the weather got hot (25+ Celsius, 77+ F), it started hunting at idle after a long run. It would sit around a 1000rpm and occasionally hunt from 600-1100 or so. This was only the beginning of last week.

Then yesterday, when it hit 30 degrees (86 F), I had a long run home (I commute about 115 miles each way to my Air Reserve unit) and 6 miles from home, while cruising at 65mph on cruise control, she shut down on me. Temperature normal on the gauge, oil pressure good, just coughed, stuttered and spluttered to a stop.

When I tried to re-start, it would cough, splutter and then quit again. 15 minutes later, it fired up and ran for 2 minutes, then went on to about 5 cylinders for 10 seconds, coughed and spluttered a few times and gave up.

Dad came and got me (fortunately we ranch together so the shop is in his yard, only 3/4 mile from my house). This morning he started it and it ran just fine for 5 minutes.

I got home this afternoon, started it and warmed it up and pulled the ignition on / engine off codes and got a 172 (HEGO (HO2S) lean) and a 556 (Fuel pump relay primary circuit fault). Both out of continuous memory. I also showed a 539 but that's only because I forgot the A/C switch on while I pulled the codes.

I haven't had a chance to put my pressure gauge on the rail to see what the idle fuel pressure is like but I suspect the fuel pump might be on the way out. It's the original pump with 190,000km on the car and 21 years so I wouldn't be surprised.

I'm planning to put a 190lph in anyway to feed my engine rebuild but what else, apart from fuel pressure, should I check? I'll change out the fuel filter anyway so that'll be out of the equation.

Stew