Very odd engine problems

**LifesHarlequin** · 12-20-2006, 07:41 PM

Alright, first post, but I was referred here by a friend, so here it goes.

I have a 1995 z28 with 63k miles on it. Other than an upgraded valve train, including ported and polished alluminum LT1 heads, the car is stock. Recently, I hired a shop to make the valve train upgrades as I had a valve cover bolt snap off flush with one of the cylinder heads, and I don't have the tools or the shop or the time to peform large mechanic jobs from my small apartment. During the repairs, the opti-spark cap was also found damaged, and only the cap was replaced. The car has yet to be dyno'ed or tuned for the upgrades as a problem arose that simply baffles all the mechanics thus far.

The car will run normally as long as the coolant temperature stays below 190 degrees (in other words, the engine stays cool) but after about 30 minutes of driving on a normal day, the temp will finally pass 190, and this is where the problem occurs. After the engine has heated up, if I try to throttle the engine too quickly, the engine will almost die, and will die unless I instantly pull off the throttle and let the engine catch itself, usually at 1100-1300 rpm. I have to from then on baby the throttle to keep the problem from re-occuring. This has become a real issue on long trips.

Current suggestions have been to replace the O2 sensors, clean the MAF sensor, replace the Coolant Temp sensor, or have the computer tuned to match the valve-train upgrades. I'm currently on deployment in the Middle East and will be able to perform any maintenance once I get back to the car in a couple weeks, but I'm looking for any and all suggestions before I get back and start spending money on replacement parts.

I also plan on a complete exhaust replacement and upgrade when I return, most likely with Mac exhaust. The O2 sensors will obviously be replaced at this point, and of course, the car will sound very nice *grin*. I'm looking for an easy 300hp at the rear wheels when I'm done. Eventually (approximately 3 years) I'll be putting an LS6 or maybe just a 383 stroker kit, and perhaps that new 6L80E transmission into the car along with various suspension upgrades, and turn it into a streetrod. But thats for another thread.

Thank you in advance for any input!

**Hi-Po** · 12-20-2006, 08:08 PM

well.... your coolant temp sensor is a thermistor. Your PCM uses info from sensor based solely on coolant temp to control, fuel delivery, igintion timing, idle speed, Torque CC operation, ERG, and usually the cooling fan. It is possible that your Sensor is bad and is causing your problems. Maybe. I guess i have never heard of a CTS causing this but im sure its possible. Before just replacing it... you can test it with a DMM and make sure.... save money by not replacing good parts.

**davebowtie** · 12-23-2006, 12:34 AM

I'm curious what u consider "upgraded valvetrain"?
was there a cam swap involved... it may be wreaking havoc
trying to survive on the stock pcm tune...
what do u think?

**YoMommasTA** · 12-23-2006, 07:25 AM

try insulating the fuel lines and fuel rail under the hood and check the fuel pressure as the problem is accuring,i'll bet your fuel is boiling in the rails.Classic symptoms you discribing.

**LifesHarlequin** · 12-24-2006, 07:38 AM

stock cam, ported and polished heads with dual coil springs and roller rockers. Cam will be done soon, but the issue occurred as I was creeping up on deployment. I'm honestly leaning toward PCM tuning because there is no fluxation in any guages, and the problem only occurs when I try to quickly throttle the engine after a certain temperature. Could the ported heads, even though its only slightly ported (2.0 and 1.6), be allowing an odd fuel air mixture into the cylinder, and after a certain temperature, the PCM simply doesn't know what to do?

The current plan is to replace the exhaust from headers back as soon as I get home, and test my sensors, but no matter what, the car is going to be tuned simply to see what I can pull out of the upgrades I've already done. If nothing else, the dyno with LT1 Edit should be able to isolate if a sensor is going bad or the PCM needs programming, correct?

Oh, something I forgot to relay. I have no Check Engine light, but it did appear randomly on a day just after the issue appeared, but it was gone within 1 minute. I went and read the code, and it translated to an EGR Valve Operation error. Again, I know the PCM controls the EGR valve, and could this alone cause the problem? And if so, why would it not affect anything before 190deg Coolant Temp?

*note: this car's final purpose is to remain an everyday driver on the street, so at best a street rod, so exhaust and cam is pretty much all that is left before I reach the power I'm looking for, and the only other upgrades will be in the suspension. I'm aiming to have the car repainted and I'll be renewing the interior in a year or so.*