V's Vette

After searching and finding nothing for a problem like this...

When my '89 modified L98 w/AT (K&N air filter, Superram intake/base, ported OE heads, ford 30# injectors, aggressive Compcams cam, new pistons/rods, chip from PCM for less, free flowing cat, stock MAF and TB) will do just fine while idling for a minute or so and then suddenly stumble/hesitate like it's going to die then gets itself going again. This happens every 2 min's or so (SWAG).

Here's what I've done in the past few days:

Replaced a very stuck IAC valve and set it per the Helms manual
Checked the timing (at spec's)
Set the TPS for .54
Set the Minimum Air to about 400 rpm in drive w/IAC unplugged
Put on MSD dist. cap and rotor
Thoroughly cleaned the Throttle Body (completely took it apart)
Quick check for vacuum leaks with propane (not thoroughly)
Also, ran 3M's strongest fuel sys cleaner last tankfull.
Checked fuel pressure - it's fine both engine off and on

The car didn't used to do this and it started right after I put on the cap and rotor. any ideas?

RonsRed88

A couple clues are the same as I had with my '88. The first few minutes the car idled fine. After it started to get warmed up the problem started and was mostly noticeable at a stop with the automatic transmission in gear. Take it out of gear and the problem wasn't nearly as pronounced. When cruising around 35 with the accelerator just barely off idle the car felt weird like it'd lose some power then pickup, then lose it, then pick up, etc. My problem turned out to be a vacuum leak. I couldn't find it with propane or water so at first I tried everything else like you have. Eventually I used carb cleaner being very careful not to spray anything that my contain a spark and discovered I had a leak in one or two places at the passenger rear of the intake manifold. It could have been the lower runner, but tightening it didn't have much effect. I was going to replace my fuel injectors anyway so when I had the top apart I checked the manifold bolts and the two above #8 were loose. I just tightened them up and my problem went away. The reason I went back to suspecting vacuum leak was because I ran datamaster to scan what was going on and found I was running very lean and the IAC was moving in and out to try and compensate. The IAC didn't start doing this until the computer went into closed loop. When the IAC closed the engine would stumble and then the IAC would snap open again and the engine would be revived. A few times the engine would actually die. Here's a screen shot of the datamaster data below taken when idling at a stop with the car in drive. You can see in the histogram window (bottom left) it was running very lean (green). In the graph window you can see the IAC moving in and out rather consistently. Notice when the IAC line is at the lowest point the RPMs drop and then the IAC opens and the RPMs return. Also notice the O2 sensor volts are consistently low.


I hope this helps,

Ron

V's Vette

Very helpful and impressively detailed response! I'll go to work looking as thoroughly as I can for a vacuum leak. I'm kind of nervous using the carb cleaner so will probably stick with propane and hope to get a 'hit.' I'm also going to look into the datamaster info for my use, too.

Thanks a lot!

V's Vette

I've checked for vac. leaks and found none using propane, trying smoke but not being a smoker I, like old Prez Clinton did not inhale, couldn't get much into the system and carb cleaner in a couple of suspicious spots. I also checked the resistance on the injectors and they are all within .20 of each other, cold and hot so I don't think they are an issue (they all sound good using a stethescope).

While watching my scanner with the car fully warmed up I noticed that when the engine would do its hesitation thing the IAC counts and the Integrator staying exactly the same as they had been when it was idling just fine. And, no TC's are being set, either.

Any thoughts/advice?

Joe C

not trying to be a wise *** or anything, but there's your first key piece of information - go back to the original cap and rotor, and see if the problem goes away...

keep us informed -

V's Vette

what would you check and how would you do it?

RetiredSFC 97

Same crapping thing happened to me. it's hard to explain but mine will be like a slight stumble or hiccup. Out of idle running down the highway it'll run fine. It won't do it till it goes into closed loop either.

It'll go prrrrrrrrrrrrrrrrhicupprrrrrrrrrrrrrrrrr rrrrrrrrrhicuphicupprrrrrrrrrrrrrrrrrrrr rrrrrrrrrrrrrrhicupprrrrrrrrrrr

I didn't save my old stuff like a DA like I usually do so I can't replace them. I did recheck everything though to include spark gap and everything seems fine.

The one thing I am going to try. I didn't replace the coil so I will replace that. I also read that a bad ground to the coil can do this.

I'll make a deal with you. I'll post up if I find it, if you will also.

V's Vette

Excellent deal...I'll take it!

lopezisgod

after puting a new motor in.. mine is doing this

mines a 86 vette with 87 motor..(roller

cold start/ egr/ smog delete.. in comp and all blocked off..

mine i cant even get mine to set base at 400 in drive.. stalls out.. and idle gets incredibly rough and just dies out... i gonna go over the whole motor again.. check iac.. see if she moving.. check base timing, clean my whole throttle body out.. check for vac leaks..

if i find anything ill let u know.. but i will as well be watching this thread..

V's Vette

I need to put in a new A/C clutch...I wonder if it's seizing every so often for a second or two and holding up the engine? Can this happen?