erratic idle/consistant miss

[tisilrad]

well i have driven it a few short trips and think i have all the fuel leaks sorted out (hopefully?).... still seems to want to run a little hot as one of my fans does not kick on? I know it works because i ran hot to it..must be the thermostat control(i have an aftermarket controller for one of the fans and one fan on manual) My new big problem is a bad miss in the engine and an erratic idle... i have 18-22" inches of vacuum at the intake manifold and the spark plugs have very few miles on them...Bosch platinum..should i go to Bosch supers? I noticed that the miss gets worse after driving around for a while...i am currently leaning towards replacing the coils because i think the ones currently on it are orig. coils from 1987!..the car has only 84500 miles on it so i doubt the p.o. changed them... the miss is bad enough to really prevent any spirited driving i need to take some exhaust temps i think and maybe narrow down the problem? any suggestions guys?

[aluminum]

Like any car you start with a compression test . That tells you about the Valves and Rings.
Always the first thing you do.
Nope not a scope A gauge A Manual Compression Test .
I suggest a Expert ( with a gauge ) because you don't most likely understand how to read a Compression Test > For example a low reading may be rings not Valves > so then a Wet Test is in Order. Oil in cylinders then retest > if it comes way up then its rings > stays down Valves.
************************************************** ***************
Most Likely the Rings and Valves are fine >>>> But that's where you start >> saves time and money >> Then you move on Vacuum Leak test > Injector Testing > Electrical Testing > Fuel Testing > Get the Idea ?

[neil30076]

Before someone on here says it - have you checked all the grounds, usual test points, etc, etc, ec...... Don't going pulling everything and anything until you have checked the basics - You have good plugs, pull, clean and re-seat the coil low voltage wires first, then pull, remove the brass/green residue on the coil high voltage wires - then report back.......

[davetr]

my car runns th same ... i know i have vacume leaks at my intake runners and at least one of my rubber connectors is leaking ...

[MPDano]

Originally Posted by davetr

my car runns th same ... i know i have vacume leaks at my intake runners and at least one of my rubber connectors is leaking ...

If you know this, then fix this first before buying all the other troubleshooting stuff. Sounds like you need to do an Intake Refresh. Replace hard hoses and ALL vacuum lines. Also, replace all those rubber couplers, they are inexpensive and vital. If you haven't replaced your heater coolant valve, replace this and the small hose as this is another issue while your in there.

[tisilrad]

well I have run a compression test in the past and it checked out ok, it was running fine when I shut it down 3 years ago prior to the fire and I have no reason why it would all the sudden lose compression..but I will retest it anyway as I still have to change the plugs.My vacuum level is fine ...I am hoping it is ignition related. All vacuum hoses are new...all of them...all grounds have been cleaned and coated with anti corrosive grease..the ground wires at the battery and the main engine ground are new 0/1 cables... this miss seems to get worse the more I drive it through my test runs? By the end of my last test drive last night it was missing badly.:P quite odd? I still need to change the spark plugs and guess I will do that today?

[jstobo]

Besides a vacuum leak, dirty injectors cause the same problems. If it has sat for 3 yrs then I would guess the injectors have old gas varnish built up in them. Pull them and have them cleaned. I couldn't believe the difference it made on my 85. I have a mechanic who has a ASNU machine so I had them done locally. But there are many online sites.

Also if the MAF is not working the computers go into limp home mode and the car runs very rich but very rough. If you have black smoke in your exhaust, then your MAF may need rebuilding.

Hope this gives you some ideas.

[tisilrad]

I found part of the problem...one of my plug wires was not making a good connection at the plug head..that fixed a large part of the miss though there still seems to be some light intermittent miss that I cannot seem top pinpoint. It still acts like it wants to die and surges at idle AFTER being driven around for about 30 minutes. it may be the MAF but I need to run some more checks... The injectors are the originals I am sure though they were actually removed from the car and did not set with gas in them...seems to me like injector number 3 is not possibly pulsing like the rest..maybe I need new injectors but they were all ohmed out and passed..maybe dirty? I purchased some starting fluid to check for vacuum leaks but I find it interesting that it maintains manifold vacuum for quite some time after being shut down? I still need to change the plugs too...
The fault indicator light has turned out to be the passengers side tail and turn signal bulb socket..it is corroded and I have no stop or tail lights on that side...the fault light goes out when I reset but comes back when I hit my breaks...easy fix, I need to get a new pigtail for that side.
My blower motor for the HVAC system is also not working...the blower works on defrost but not any other settings so i must assume it is the usual..relay or resistor group..hopefully get this sorted out tomorrow.
Overall i am quite pleased with the results of my rebuild...I expected some problems and hope to get these ironed out with help from you guys
Thanks!

[neil30076]

May not be either relay or resistor group, in 30 yrs of ownership never had to fix either - Do a lot of searching and reading on the blower issue - it is not an easy one to fix - and you need to be really methodical about it - do a search on r list for blower and you will get more that you ever wanted. If you want porsche trouble shooting guides PM me.

[MPDano]

I actually had the same erratic running on my Mitsu. I had all my Injectors Serviced and Flowmatched and they passed with flying colors. Once you ran the car till operating temp, this is when one of the Injectors started to fail, causing an erratic run. Took me forever to track down. Replaced the faulty injector and it has ran perfect ever since.

http://forums.pelicanparts.com/porsche-928-technical-forum/546641-fuel-injector-testing-cleaning.html

[MPDano]

Here is how I found the faulty Injector:

98 GST P0303 and P0203 Codes - Please Help - DSM Forums: Mitsubishi Eclipse, Plymouth Laser, and Eagle Talon Forum: DSMtalk.com

[924CarreraGTP]

Or...... Replace your plug wires altogether if one has recently given you problems. Start fresh. Restore the ignition system. Use OEM german parts. I prefer Denso Iridium plugs but thats my opinion. Bosch plus 2 and plus 4s are cool. When you find one problem in that system, you might as well check the rest of it. One of my plug wires used to blow off until I realized the plug was loose.
I'm going through some problems with my '83 heating up too. I keep the A/C on because it doesn't work and I disconnected the belt for more HP anyway. This keeps the electric fan on all the time and keeps my engine cooler. It could be any number of things. Air flow, fan, thermostat, clogged radiator, or the sensors. If you wanna throw used parts at it, let me know and I can help you for a decent price as can some of the rest of us.

[tisilrad]

Well the plug wires are actually new...i made them using factory ends and crimping new terminals onto the ends of the wires to screw in the connectors..one of my crimps did not hold...one of the first ones i had done.
I still need to change the plugs...it still idles funny after warm up..i am going to go over the vacuum system today and try yet again to change spark plugs. I also need to fabricate an undertray and front splitter...i have a carbon fiber splitter from a NASCAR stock car to install...i think it will run a litt

le cooler with these installed..though it is not overheating as of now it still runs right below the 3/4 mark on the gauge. I will try to take some pics or video to give everyone an idea of whats been done.

[Mrmerlin]

the home made plug wires really leave a lot of possibilities for failure,
I would consider getting a ready made set and plug and play,
seen this a few times

[tisilrad]

well crap, got home today to set my fuel pressure and do more diagnostic and the car will not start..no power to the fuel pump...fuel pump runs with direct hot, swapped all three relays (though the EZK, LH and fuel pump relays were all new) to no avail..i fear the ECU is now dead...i guess i need to do a little more testing but i feared this would happen, as i felt one of the resistors on the computer board looked overheated before i installed the unit...it may be the reason i have been having idle speed and stalling problems in any event. funny it ran pretty good last night and today..nothing

[neil30076]

Contact Rich at Electronik Repair offer - free testing - 928 Owners Club Forums he can test the boards for you. Join the 928OC and he'll do it for free! If you do contact him, say I sent you

[davetr]

mpdano ... i agree w you completely .. i have all new intake gaskets, new rubber boots .. all fuel hoses etc .. everything i can think of ... i will not drive th car till i can get to it . maby this weekend ? i will replace everything that is needed .... and since i dont know th history of th car i will also be doing th timeing belt as well ... also il proly do th water pump while im in there

[MPDano]

Originally Posted by davetr

mpdano ... i agree w you completely .. i have all new intake gaskets, new rubber boots .. all fuel hoses etc .. everything i can think of ... i will not drive th car till i can get to it . maby this weekend ? i will replace everything that is needed .... and since i dont know th history of th car i will also be doing th timeing belt as well ... also il proly do th water pump while im in there

Spoken like a true 928er

Also, while your in there, you should send out your Injectors to get serviced at Witchhunter. They come back looking brand spanking new, plus the will give you a spreadsheet on how they were before and after. They will also be flowmatched and nice new seals.

[tisilrad]

I have a completely new set of flow matched injectors i have yet to install.. i wanted to see how it ran with the stock injectors before upgrading to my gen 3 injectors..all the hoses and gaskets on the top of the engine are new...

[MPDano]

Originally Posted by tisilrad

I have a completely new set of flow matched injectors i have yet to install.. i wanted to see how it ran with the stock injectors before upgrading to my gen 3 injectors..all the hoses and gaskets on the top of the engine are new...

gen 3 injectors