steeeve_c

Hey everyone, I have an 85 Trans Am with a bone stock 305 TPI under the hood. Everytime I start the car after it's been warmed up, it likes to "flare" or raise instantly to 1500 rpm's for around 3-5 sometimes close to 10 seconds and then lower down to idle. My TPS was replaced a few years ago and is at .54v and steadily raises voltage as you open the throttle more,. IAC works fine. I know this because I put a new one on and nothing changed. MAF is a few years old. O2 sensor is a few years old. Plugs, wires, distributor cap & rotor are 5 years old. I've driven the car 5,000 miles over the past 5 years. I have an aftermarket ebay "cold air intake" on the car. It still started weird with the stock air box. I changed my coolant temperature sensor today and the car seemed to respond much better whenever I gave it any throttle. I changed the fuel filter a few weeks ago. I've checked numerous times for vacuum leaks. I couldn't find any. My cruise control works perfect, so doesn't that mean the vacuum pressure is fine? The fuel pump was replaced around 4 years ago. When cold, it starts and idles at 1100 - 1200 rpm's before backing down after warming up. I'm honestly out of ideas... The car has 42,000 miles on it and it runs strong. I've even replaced the throttle body gasket a couple years ago. I tested the ohms on all injectors and got the same reading. 16.7 ohms. The readings were all spot on though. I also replaced the PCV valve and grommets 2 years ago. I'll attach a link so you guys can see. I'd really appreciate the help..

Chopski

iTrader: (1)

Mine does pretty much the exact same thing. Hopefully someone knows the answer.

TOM305T/A

yup i have a 350 tpi. when its warm it still goes to 1500 when i start it up, 10-15 secs its down to 600 rpm

calidude

Reviving this thread as hope someone else is seeing the same as this is exactly what my car is doing. I was planning to check if the cold start injector and the cold start switch may be the cause of this.

Would appreciate if someone else can confirm if they have seen similar and how you have resolved?

calidude

Replaced the coolant temperature sensor and no change except idles smoother at 600 rpm.
Revs to 1500 rpm on warm start and then falls after about 8-10 seconds, sometimes to 600 rpm sometimes to about 800 rpm. No codes from ECM, also unplugged cold start thermal switch and no change.

Going to check timing next….

southbay08

stupid question....did you test the injectors when they were both hot and cold???

calidude

Injectors were rebuilt less than a year ago by RC Fuel Injection locally here in Torrance, CA so I don't suspect them...

Now car is throwing a code 34 as just popped up today as I was taking it for drive, it was hesitating a bit until it warmed up too. The Check Engine Light points to the MAF and will need to troubleshoot and run tests next I guess as was reading this too - https://www.thirdgen.org/forums/tech...-mass-air.html

Appreciate any help

Reddragon88gta

Try cleaning the MAF with CFC MAF cleaner and check the relays. If that does not work start trolling eBay for a 85’ only MAF as it was a 1 year only deal as you probably know.

My original MAF was tough to trouble shoot as it would only act up once heat soaked and cause a rich condition.

calidude

Thank you my friend, solved the check engine light and code 34 issue. Huge improvement in performance and drivability too, the MAF is crimp clamped onto the air cleaner assembly but sprayed it at least from the boot side.

I’m still trying to troubleshoot this warm start and if it’s normal for an 1985 or not? I would expect it to not jump up in rpm so much on a warm start -

Reddragon88gta

I watched your video, that does not seem correct that it would idle up that high on a engine that was at operating temp. You have already looked at the usual suspects but I would try to get a scanner and run data of a hot start and see if there is anything off. It could just be a quirk of the 85s but I would check the wiring of some of those sensors like MAF and CTS too.

That was the only way my faulty MAF was discovered, it would heat soak and on warm start, only warm start BTW, I would get smoky start ups like it had too much fuel. It was verified that when the MAF would heat soak its readings were way off. New used Bosch MAF solved it.

If you need specific MAF part numbers here they are:

calidude

Thanks yeah all the wiring is intact and California car so missing the humidity that normally destroys things. Very clean connectors and contact points, I replaced the wire looming and re-taped with 3M as the old stuff just crumbled. Thanks for sharing the MAF part numbers but I have renewed confidence in it since cleaning. Wish some other 1985 folks that would chime to confirm if this is normal behavior for this year.

I get some smoky starts at times but believe it's oil seals for valves, another fun job

Any recommendations on a scan tool?

Reddragon88gta

I like the 1320 electronics Bluetooth scanner that works great with an old android phone. Last I heard he was sold out but it’s a lot less money that the snap on mt2500 or the brick as it’s commonly known. The thing has paid for itself easily for diagnostic work on my car. I use the aldl droid app which is easy to use but the aldl scan tool app is another good one I’ve heard but I don’t have any experience on it.

calidude

Thanks will look at those..

Checked and set base timing as was advanced a bit, purrs like a kitten at idle now.

Going to ping a coupe others with the 1985 TPI to verify warm start.

I combed the 1985 shop manual a bit and read starting mode the ECM checks coolant sensor and TPS to determine proper air fuel for starting with a cool range at 1:5:1 -33F to warm at 14:7:1 200F

run mode on engine first started and rpm above 400 system goes open loop and ignores O2 and calculates air/fuel off coolant and MAF

…appears the run mode stays in open loop until O2 is hot enough, the coolant sensor is above specific temp, specific time has lapsed then goes into closed loop so it can get into a 14:7:1 air fuel ratio

believe this is the cycle I’m seeing on both cold and warm so it’s like the ECM is getting wrong data or not functioning properly?

I bought an O2 sensor to try but the original body looks smaller. May swap and try that too.

What is baffling is no codes are being thrown

Reddragon88gta

The O2 sensor is a good idea, they wear out over time and can get lazy.

No codes is so typical with these cars, most of my annoying issues had no codes either which made having a data scanner so important.

calidude

Replaced the original AC spark plug O2 sensor as it was written on, bought a new GM and broke the plastic wiring clip, soldered in a new one from Standard Ignition. It’s noticeable as feels more responsive.

No change with warm idle, for the heck of it also replaced the IAC as was original with a new Standard Ignition one. Again no change…

@1985_IROC or @dan5 have you guys seen similar with your 1985 model? My video is above showing the scenario I’m stuck trying to solve.

Appreciate any help as frustrating @GeneralDisorder any thoughts?