Help me people, my IROC is posessed. It goes something like this:

About two weeks ago it suddenly started guzzling fuel like crazy. I used to get about 15-17 mpg in city traffic before but now it's dropped down to about only 10 mpg. This rise in consumption was also accompanied by overheating. It was hot the day it started overheating (high 90s) but the car kept boiling over like never before. Everything checked out fine. I had a 180 t-stat then. The car probably has an aftermarket fan switch setup because one fan starts as soon as I switch the ignition on and the other one comes on once the engine is running. I thought my fans were not working but sure enough they were. I even soldered a loose splice in the wiring harness to make sure they wouldn't shut off without my knowledge.

Before this whole overheating drama, engine temp used to stay steady between 160 and 180 even in the slowest traffic and hottest weather, but now it hits 220 even with a 160 t-stat that i put in there after it overheated; all while the fans are on full blast! The strange thing is that it runs too cold on the highway now. That never happened with the 180 t-stat. Now the temp goes down to about 130 after a five minute run on the highway. That's way too low.

I know it's not a question of airflow. The air dam is still intact, radiator is not clogged from inside, nor obstructed by leaves, plastic bags, or any such thing from outside. Don't have a leaking head gasket either, or dieing water pump. In fact, the water pump and radiator cap are new. The fans are also not dieing. They are still running strong. Neither are the temp gauge or sending unit faulty. If they were, the coolant wouldn't be boiling over.

What could be causing it to rnn so hot in the city? What gives? I'm absolutely clueless. All the cooling system components have checked out fine.

I've never been a fan of the 160 t-stat because it used to make my car run rough but now that's the only t-stat that will prevent the car from overheating, but makes it run too cold on the highway.

Now the fuel consumption problem which may or may not be related to the overheating problem. About the same time as it over heated, or actually a week before that, my gas mileage went completely to the dogs - down from 15-17 mpg in city traffic to barely 10-12 mpg. My old O2 sensor was worn out so i put in a new one but it hasn't made any difference whatsoever. I also checked out the coolant temp sensor today and it checked out fine - resistance values indicated it was reading a little hotter than actual engine temp. But that would cause the ECM to leanout the A/F mixture rather than enrich it.

There are no fuel leaks either. The injectors might leak a little bit though because I sometimes have a hard time with cold starts. It seems to fire but doesn't catch. Usually, i can wait a minute and try again and it fires right up. But a mild case of leaking injectors wouldn't cause gas mileage to drop nearly 40%.

There's also another thing I noticed about the O2 sensor circuit - disconnecting the sensor makes no difference to the engine. The SES light doesn't come on, and sure enough no codes are set. Not with the new sensor, not with old one. I tried this many months ago and I thought maybe after a new sensor the circuit would start working right but it's still the same. It even goes into closed loop with the O2 sensor disconnected. At least I was still getting 17 mpg then . Now it's totally FUBARed. I can't figure out what's going on with it and it drives me nuts.

Does anybody have any idea why it's acting so sh!tty. Where should I go from here? I've already ruled out all the obvious reasons for overheating but I'm still pretty confused as to what i should do about the fuel consumption problem. Please people help me! I'm running out of options!



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'88 IROC 305 TPI
Crappy 700R4 slushbox
Gutted airboxes
180 degree T-stat
Advanced base TPS voltage
Relocated IAT sensor
Momo steering wheel (gotta luv it)
Ram-air setup coming soon
Flowmaster muffler (puke)
Taylor SpiroPro wires
Accel cap and rotor
Ported plenum
Kills: '94 Z28, Olds Aurora V8, bunch of Mustangs, T-birds, ricers, and others who assumed a 12 year-old car would be too slow.

 

You aren't get any codes at all? Can you get any codes? (does the ses light flash at all?) Not having the o2 sensor plugged in should throw a code.

 

Well man that sucks. looks like you went over almost everything but fisrt things first. since you have changed the thermostat, waterpump, and no blown head gaskets. have you performed a compression test to verify that that head gaskets are not blown/cracked head? it could be air in the cooling system creating hot spots and causing this problem you having.

 

Is there a working bulb for the SES? Bad ecm maybe? When I had a bad ecm the car ran like it was possessed and also ran too hot but I also had a couple bad plug wires at the time...

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I ROCK


...so you don't have to.

 

Clogged catalytic converter. Most muffler shops can test them with a pyrometer while the engine's running.

If the case temp gets way hot, and the exhaust pipe just downstream of it is way cool, you generally have a diagnosis of failed cat.

-Kevin

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1982 Z-28

See http://www.mycar.net/mafb/registry/detail.cfm?id=276 for details

 

Thanks for replying guys!

Well, I haven't had the opportunity to do a compression test but I do know that there is no smoke or antifreeze smell coming out the exhaust pipe at any time. The SES light has a good bulb - the only code it would show was a 12, which indicates diagnostic mode. I don't know about the cat. I'll have it checked out. It very well could be the cat because I can hold the exhaust pipe and touch the muffler with my bare hands without any discomfort while the car is at operating temperature.

I have an update. I took it to the mechanic, the only competent one in my area who knows anything about these cars, and told him exactly what was going on. I didn't see what tests he performed on it but when I got the car back from him he said that everything showed up perfectly fine on his computer except for the MAF. I guess that he could hook up his computer to the car and test it shows the ECM is probably good too.

He said the MAF should read 8ppm at idle but instead it was showing 11ppm. The ECM might be compensating for the higher reading by dumping in extra fuel. Would it make such a dramatic difference in gas mileage? This isn't even a warranty item . As for the temperature, the CTS indicated engine temperature to be about 207, while the temp gauge never went above 160 while it idled for an hour. On the mechanic's insistence I got a new temp gauge sending unitand put the 180 t-stat back in and now the temp never goes below 230, even out on the highway in 80* weather. He also told me that while the cooling fans were running all the time during the test, the ECM itself never switched the fans on. In other words, the ECM didn't see a condition for the fans to come on, and they've been rigged to turn on a lot sooner than they should. In fact, I don't think the ECM controls the fans any longer - one fan starts running with the key on engine off, and the other one starts too once the engine is running.

Now I only have more questions instead of answers. How could the engine be at 207F with both fans on, and a 160 t-stat? How come the temp gauge is now indicating temp to be at 240 while the CTS shows about 40 degrees less? Is the temp gauge itself also screwed up, or do I have the wrong kind of sending unit?

I drove the car today for a while after changing the temp gauge sending unit and t-stat, and it feels perfectly fine. The temp gauge needle sits way up at 230 but the car behaves pretty reasonably except for a stumble around 1200rpm during easy take off from a stop. All the power is otherwise there. It will still haul tail like it used to before.

I was also thinking that because the mechanic probably performed the tests while the car was idling, he got an incomplete picture of what's really going on with the engine. Even the most mistuned motors will barely sip gas at idle. Part throttle is where all the action is. I don't think he drove the car with his computer hooked up to see what it would do then.

The question is what should I do now? I'm completely clueless. Could the waterpump have gone bad already? It's an autozone unit with lifetime warranty. Doesn't leak or make any noise. Could the radiator have suddenly choked up really bad? Judging from the flow inside it, I doubt it. Revving the engine with the cap removed purges pretty copious amounts of coolant from the system. The level first drops about an inch or two then rises back really fast once I let go of the gas. BTW, the coolant is also mostly water now.

Any more ideas about what I should do now, other than pushing the car off a cliff? Where are you Vader? I need your help! Maybe you or other thirdgen gurus might have some magical pearls of wisdom for me .

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'88 IROC 305 TPI
Crappy 700R4 slushbox
Gutted airboxes
180 degree T-stat
Advanced base TPS voltage
Relocated IAT sensor
Momo steering wheel (gotta luv it)
Ram-air setup coming soon
Flowmaster muffler (puke)
Taylor SpiroPro wires
Accel cap and rotor
Ported plenum
Kills: '94 Z28, Olds Aurora V8, bunch of Mustangs, T-birds, ricers, and others who assumed a 12 year-old car would be too slow.

 

Man try replacing the Radiator Cap. I know it sounds stupid but that's what was wrong with mine and it was doing the same thing. Not sure about the fuel problem though.

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Trans Am:
(Engine)305 TPI
180 Degree Thermostat
JET Fan Switch
TB Bypass
Accel 300+ 8.8 Racing Wires
Accel Super Coil
Bosch Platinium Plugs
Custom Ram Air with K&N
Removed Emissions Equipment & Cat.
(Stereo)Full Kenwood Excelon System
See my Ride:
https://www.thirdgen.org/rides/index...ew&rideid=1324
Member at kyfbodies.org

 

When my ecm was bad I believe it also showed 12. The scanner gets its info from the ecm, and the ecm gets its temp info from the cts, so assuming both sensors are good (since they're new right?) then either the guage is wrong or the ecm is. There's a reason the maf isn't on warranty lol. But that's probably not the problem - you can try unplugging it and the car should run much much better if the maf is a problem. On my iroc the ecm turns the primary fan on at 215 and i have a switch that turns on the secondary at 203, which means the both come on at 215. If you have a stock switch, your primary should come on at 215 and your secondary at around 235. All these things tend to point to ecm, but there's even another thing, that if all was ok except for running hot you should be getting BETTER gas mileage. I'd definitely still check the cat for sure, and maybe the cooling system, but it just sounds like the computer to me, although I'm not a licensed tech or anything. Did you say your mechanic tested the ecm and said it was ok? If the computer is fine then I'm clueless except maybe to guess an internal head gasket leak. Good luck.

 

Well I can also say that disconnecting the O2 sensor will also not through a code and this is at operating temperature. My gas mileage is not were it should be but is better since I changed computer and new chip. My old chip in the car was burnt by a mob here in Australia nd it was a rough job. Half the tables were left blank all this was doen when they converted my 86 to SD. Since the computer swap which they did not do the car is now getting way better gas mileage but still will not throw a code when O2 disconnected. Don't know what it is, and yes I too get a code 12 when I go into diagnostic mode meaning all ok.

Sorry can't help much but I have the same problem as you.

 

The cat checked out fine too. Exhaust tech checked it and said it was just fine. I finally decided to fill it up and see how long it would run and here are the results. I averaged 16 mpg. That's 225 miles in 14.062 gallons. Out of these 225 miles roughly 70 were on the highway and the rest in stop and go city traffic. Do you think it's about OK for a stock TPI 305? I have a feeling it really guzzles in the city but makes up for it on the highway. Or maybe I'm just paranoid...

I noticed another thing yesterday. The engine was running way rich at idle. I could smell it in the exhaust. The only time it smelled like this before was when I had the 160 t-stat in there 2 winters ago.

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'88 IROC 305 TPI
Crappy 700R4 slushbox
Gutted airboxes
180 degree T-stat
Advanced base TPS voltage
Relocated IAT sensor
Momo steering wheel (gotta luv it)
Ram-air setup coming soon
Flowmaster muffler (puke)
Taylor SpiroPro wires
Accel cap and rotor
Ported plenum
Throttle body bypass
Kills: '94 Z28, Olds Aurora V8, bunch of Mustangs, T-birds, ricers, and others who assumed a 12 year-old car would be too slow.