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misfire on renix 4.0 on hot startup

T

Tito_73*

NAXJA Forum User
Location
California
O.K. here is the run down sorry if it's a long one. My 89 will only miss when it is warmed up and only for a short while. Maybe like 20-30 sec. I have rebuilt the engine and replaced the cps, iat, ect, map, knock, fuel pump, injectors, fuel pressure regulator, coil, well you get the picture. I have a snap on mt2500, so the other day I was viewing the sensor data. I found out that my o2 sensor took a dump. I bought another one, bosh# 12009. So anyways it still does it, I have o.e.m. cap, rotors, wires, and plugs (champion copper core). I know I didn't dent the flywheel when I pulled the engine because it would miss all of the time. Everything looks good I thought the engine had air in the cooling system but my ect pid says 203 deg. with the key on engine off, my tps voltage is a .7 which is normal. I don't know if my sync sensor is taking a crap because it has no problem starting up. my icm seems to be working because it has no problem starting up it just misses for a while when it is started up when warm. Sorry for the long post but any advice is helpful at this point. Thanks NAXJA
 
I know this may sound funny, but check the gap on your plugs. (copper Champions right?) A plug gapped too tight caused a similar problem on my rig. Ran fine cold, but ran rough like a miss on a hot start.
I will never put a plug in without checking the gap again!:doh:
 
Yeah I know the scan tool is not a sure way to test the sync sensor it just changes from + to - while the engine is running. The reason i don't think it's the sync sensor is it doesn't take much time to start when hot it's not even hard to start. I know the proper way to check it would be with an analog type voltmeter. Also thanks for the advice on gaping my plugs, to answer your question they are gapped at .035. I guess I will try and figure it out when I feel motivated, at least it doesn't do it all the time and for that I should be happy. :NAXJA:
 
my 89 does it too, now I try to leave it running for quick stops. its embarrassing,especially in 4 lo 1st when on the trail with the guys. I've tried everything!!!!Its like losing two or three cylinders briefly right?
 
I've seen this on the multi electrode plugs.
 
Mine will do this on occasion when it's been run hard. Things to look at would be.

Is the heat shield still on the intake manifold?
Are any of the fuel lines too close to the exhaust?
Are the injectors smaller than before?

As long as it's not running too lean and getting hot you might try insulating the injectors with fiberglass sleeving.
 
thanks for the advice. I had the stock replacement injectors put in from kragen niehoff brand 57170 i believe, all six when I rebuilt the motor. Then I went to ford mustang 19 lbs. yellow tops. I now currently have 24 lbs. blue tops. I still have the factory heat shield on the intake manifold, and I have the fuel pressure line supported with that black plate to the intake manifold. It will kind of run a little rough feeling like it's dropping a cylinder when idling right before the electric fan comes on. It's my daily driver so it's kind of annoying when I gas up and go to restart. I have replaced my idle stepper motor the pcv harness and the vacuum harness. I don't think boring the engine .030 would cause more heat to develop inside the block causing a miss when hot. I am not sure other than that she runs great. Gotta love those intermittent renix issues. :sure:
 
tito, did you replace the head when you did your motor?I had a jeep/chyrsler guy tell me on higher mile renix heads they sometimes have sticky exhaust valves. I did a can of seafoam in the throttle body and a 1/2 can in the fuel tank and It helped breifly,maybe a couple weeks. I have a shortblock and a new head to install when the weather warms up again.I hope this fixes it, You name it I've replaced it trying to fix it!

The first time my engine did it was last december I was chained up and playing in the snow, shut er down for like a minute and started it put put put put ,then clears after some rpms and about 30 seconds.
 
Yes I did a complete long block overhaul. Completely rebuilt cylinder head, The first thing I thought was a sticking valve problem. It is so weird that it only does it when it sits hot. If I let it sit for about an hour it wont do it. So temperature is a factor. I am almost sure if I replace the icm, and the distributor along with stator (sync sensor) it will still do it. I know there is a service message dealing with replacing the cps sensor and installing a shorter jumper harness to the computer. But that is only related with a crank-no-start concern. Oh if your wondering, new fan clutch, thermostat, electric fan, hoses, and radiator, crappy coolant surge tank, and crappy plastic cap. Scan tool shows rpm drop when it misses (of course) and the computer is trying to add fuel due to the unburned oxygen passing over the o2 sensor. I hope someone has found a fix for this issue. NAXJA
 
My 89 has the EXACT same problem every once and awhile,still haven't gotten to the bottom of it.....I thinks it's vapor lock.I have learned that outside temperature is not a factor.....underhood temp is.Seems like mine only does it after a long drive,even then it only does it once in a great while.
Thats what sucks about it....you never know when it will do it.Troubleshooting this is an enigma shrouded in a mystery!!!
 
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Mine is very predicatable, I was thinking fuel boiling in the rail, I applied hi temp heat tape to everything on the rail, no change, I have replaced all injectors, pressure regulator and even installed a bmw check valve, I'm pretty sure fuel is not the problem. Can a lifter stick and not tick??Tjhe only thing I have not changed is the coil
 
As far as a hydraulic lifter sticking I would say it would have to tick since it is collapsed therefore resulting in excessive clearance in the valve train. I would think this would be most noticeable on a cold start since the oil would bleed down over a long period of time. Mine doesn't make any noise at all, like you said ambient temp is not an issue just underhood temp. The only thing I could think of right now is the manifold air temp sensor mine is new but while running with a hot engine the pid indicated around 165 to 170 deg inlet temp. I am sure this is normal since it is threaded into the intake manifold. Also the temp decreases when the throttle is opened. I guess I need to clean my throttle body and idle stepper motor. Check the manifold bolts and make sure there tight. It most likely will still do it though.
 
Oh yeah I forgot to mention I even replaced the B+ latch relay since I thought it was bad, the renix manual says it supplies power to the ecm with the key off so it can properly position the idle stepper motor for the next startup. I should check the terminals to see if they have the proper power and grounds.
 
I'd get it hot shut it off, let it sit a little while then turn the key on and depress the fuel schrader valve to check for vapor/air. It would be safer to hook up a fuel pressure gauge check it, how much/fast the pressure bleeds off too.
 
Ok I just thought of something. I checked the fuel pressure when it was hot with a gauge and it stayed at 40 psi I don't know the temp of the fuel but I can perform this test again. I am going to purchase one of those infrared temp guns you know the one that points a laser. I need to physically check the temp at the block the injectors and maybe a fuel sample. I also should use a fan to cool it off and see at what engine temp will cause it not to miss. I really want to get this taken care of since it seems that all of the renix 4.0's don't do it. Well I will try and post some results soon.
!!!1
 
I've installed a used throttle body with a new Iac and tps, as well as a air intake temp sensor, trying to battle this beast
 
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