Help Technical Gurus!

My 86 305TPi is doing some strange things when the engine is first started from cold.

The engine fires up and then starts to surge really bad from 1100 rpm down to 600 rpm and anywhere in between. This only happens for maybe 2/3 minutes and then clears. The engine runs strong and smooth after these weird idling problems. When the surging takes place the Radiator Fan kicks in and clicking from the throttle body area can be heard - Then everthing clears and runs okay.

I have a new IAC valve and the TPS/IAC valve are set correctly. When the engine is hot it idles at about 650/700 RPM in Drive and 850 in Park.

Anybody got any ideas.:confused

Any information gratefully received - BTW I have no Trouble Codes etc.

Thanks in anticipation

 

Is this a new behavior, or has your car always done this? If it has always done this, have you checked into getting the updated EPROM for your vechicle? There was a technical bulletin that outlined an updated eprom that was released for 86's, so you might want to check your date code. If you do a search for "updated eprom" you will probably find all the info.


------------------
Mike Metzler (Desert86Roc)[*] Check Out:SpeedWorldMotorplex.com[*] Check Out:Chevrolet F-Body Online Part & Illustration Manual[*] My 86 IROC 305 TPI Page (406 build in progress)

ET's @ 1250 ft[*] 14.28 @ 95.461 mph (uncorrected, NOS, no headers)[*] 15.362 @ 88.238 mph (uncorrected, headers, no NOS)
<><

 

My friends 87 iroc does something like this. His is real questionable as to why though. When he bought the car it had already had the 305 replaced with a 350. It still has the 305 injection setup though(not that they are too different). It still has the 19# injectors 305 knock sensor and computer. We do not know a lot about the engine yet. It has a slightly bigger than stock cam and is a 355 with some sort of trw pistons. He gets no engine codes but it has several problems. If the battery has been unhooked and the computer looses its memory it will run fluctuate idle really bad untill the computer learns to compensate for it. I am pretty sure that this is not caused by the same thing but it reminded me of it.

First I was wondering if he should continue to use the 305 knock sensor untill he gets the 350 chip and injectors.

Also his temp tends to run high (220 to 240 range), and sometimes the engine will surge like you are tapping on and off of the accelerator but you are just holding it steady.

If anyone knows what could cause these problems (whether it be something that was was not done durring the engine swap or just a normal maintenence type problem) I and especially he would really appreciate it if you would reply. I am hoping that with all of the knowledge and experience on this board someone will know what is causing his problem so that it can be fixed.

thanks

Ben

 

I'm pretty sure the 305 and 350 knock sensors are the same, but the ESC module next to the MAF relays is different between these 2 engines. The ECM will be the same, but the chip is different of course. The temp range is normal for a stock-chip'd TPI engine. Get a lower-temp 'stat and a custom-burned chip (the home-burned type are the best) and he can take care of most of his engine problems, assuming there aren't any mechanical or sensor problems (probably not if no codes are being set). With the hotter-than-stock cam, he'll need to play with the first MAF table, bump up the idle speed some, and work on the spark advance and power enrichment tables to get it running good.

------------------
Greg Westphal
'87 IROC 355TPI/A4

 

Here is the link to the TSB. See what you think (I know this was written with firebirds in mind, but also applies to our IROCs. http://www.gmtips.com/3rd-degree/dox/tips/ecm/prom.htm

As for you other guys: 305 and 350 resonate at different frequencies...they DO have different knock sensors. To do a 305 to 350 swap, you must at the minimum change the knock sensor, injectors (or compensate with more fuel pressure), and a 350 based chip. Otherwise, you are leaning your engine out bad at WOT and looking to burn it up.

------------------
Mike Metzler (Desert86Roc)[*] Check Out:SpeedWorldMotorplex.com[*] Check Out:Chevrolet F-Body Online Part & Illustration Manual[*] My 86 IROC 305 TPI Page (406 build in progress)

ET's @ 1250 ft[*] 14.28 @ 95.461 mph (uncorrected, NOS, no headers)[*] 15.362 @ 88.238 mph (uncorrected, headers, no NOS)
<><

[This message has been edited by Desert86Roc (edited October 23, 2001).]

 

So if he does not get the 350 chip quite yet will the 350 knock sensor work with the 305 prom. Also is it possible to turn up the fuel pressure on the stock tpi setup or not. Thanks for your help.

 

Yes it'll work, but not optimally or as well as it would with the 350 chip, knock sensor, and ESC module. You can turn up the fuel pressure if you install an adjustable fuel pressure regulator (AFPR).

------------------
Greg Westphal
'87 IROC 355TPI/A4