Hello everyone I have an 88 automatic that almost stalls when I first start it when cold. It starts right up and the the rpm goes to about 500 RPM and then back up to 1100 and bounces a bit between 1000 and 1100 until it goes into closed loop and settles at 700 RPM. My idea is that the open loop mode is too lean or too rich since this started to happen when I installed a custom chip. It could also be that the IAC is not operating as it should. Has anyone had this problem after installing a custom chip. I have some minor mods like 1.6 rockers, ported plenum and SLP runners with a flowmaster exhaust. Other than that, the car runs great even with 180,000 miles on the clock. Thanks for any ideas.

 

Cold start and warm up idle is mostly dependent upon the open loop AFR vs coolant temp and desired idle rpm vs coolant temp. Spark advance is also an important factor in maitaining idle stability.

Error in the open loop AFR or poor open loop AFR targets can certainly contribute to idle problems, this is mainly a function of fuel pressure and MAF calibration and of course, the open loop target AFR vs coolant temp.

In many bins, the open loop AFR is even leaner than stoichiometric (14.73:1) during warm up for medium coolant temps (approx 100F-160F).

This can be a problem for cars that are reluctant to go into closed loop due to headers or non-heated O2 sensor issues. You may want to try raising the fuel pressure to combat the potential for open loop lean-ness, if a chip re-tune is out of the question.

If the revs drop that low after the initial rpm flare up, there probably isn't enough throttle opening or the IAC start up park position is too low. The engine is pulling the air out of the intake faster than it can be replenished. The IAC can't really add air very quickly unless the d-part gains are increased. Even then it is subject to considerable delay.

Best bet would be to log some scan data during a cold start and idle through warm up temperatures up to normal running temperature and contact your tuner.

Also a good idea to verify tps and min air adjustments as well as ignition timing and fuel pressure (all the basics).

 

Thanks for your input tequilaboy. I know the TPS is ok at o.547 volts but I don't know what the fuel pressure is yet. You are right chances are there needs to be some changes in the AFR during open loop. Some scan data is probably needed to fine tune the chip.

 

Hi guys. I have the same exact problem. I now have a 383 with a custom chip (fastchips.com) and my 86 starts and immediately wants to die below 500 rpm. If I feather it for 30 seconds and try manually to keep the rpm around 1100, the chip will take over after about 30 seconds of struggling and surge to 1300-1400 for about 1 second before settling back down to 700 and staying there. This happens EVERY time I start it no matter the outside temperature or the coolant temp or anything else really.

I've finally gotten the moates chip burner and can actually play with Mark Mansur's TunerPro but don't dare start to change any parameters yet. I've changed the number 1 coolant fan temp on and off parameters and the chip actually did it just like I said. I had a grin the size of nebraska at that moment. I also changed the idle/temperature table telling it to stay at 1100 rpm till the engine reaches 130 degrees instead of the graduated table that was there but the behaviour I describe at the top of my post supercedes this table and the table valuies don't seem to matter at all.

What in ya'll's opinion is going on and what parameter should I attempt to change to zero in on this problem? I'm STILL trying to record data but haven't been successful yet. I'm amazed at how non trivial all this is. Complex if you ask me

Thanks from the new old guy