drchambers

I have a 1995 C4 Cab. When it's 65 degrees + outside and the engine's hot it occasionally cranks but won't start. After sitting for 10-20 minutes it will start. Recently it's started stumbling and bucking at more than half throttle and doesn't want to rev past 4K RPM. I don't know that the two symptoms are related. When it won't start it doesn't have spark. I've swapped the DME relay and replaced the crank/speed sensor with no change. I don't know if there's fuel when it refuses to start. I did replace the pump and filter post no-start. I don't really mind replacing original parts at it as it's 27 years old and I have all of the records and I plan to keep it for a while. Spares are good.

How does the ignition module work? Are the two channels separate, or can it fail such that both distributors behave similarly? I generally don't have a lot of time for troubleshooting when it doesn't start. By the time I get the tools or find some help it fires back up.

Thanks!

IainM

When were the spark plugs, wires, distributor caps & rotors last replaced?
How many miles on the car? Last service?
How often do you drive the car? 100 miles a day, a month, a year?
Some additional info will help get more responses.

You say you don't have spark - did you pull a plug wire and check with a spare plug to chassis?
Do you have fuel pressure on the rail? - check at the schrader valve.
Do you have a scan tool and if so what codes are showing (if you don't then this is the first thing to do if you plan to keep this car. Search here for scantool, there are several.
Search here for hot start problems, many threads and ideas such as CHT sensor etc.

While you can throw parts at the problem, its usually faster to test and identify the failed part first.

Oh and pictures. We love to see new cars.

drchambers

No spark at the coils, 79K miles, no codes. The temp sensor was replaced at some point, but isn't causing the no-start issue.

IainM

two coils are completely independent. Unlikely they both failed at the same time. If you take the dizzy caps off and jiggle one rotor, does the other rotor move (quick test for dizzy belt)

I take it if you jump the fuel pump at the DEM relay, you hear it whirring?

Search for ignition circuit failures here. I don't remember all the pieces that have to be present & correct for a spark to occur. Again, highly unlikely that both failed at the same time. Most likely an ECU input confusion or immobilizer snafu.
Also, pop the cover off the DME relay and check solder joints around the spade terminals, They always fail there.

drchambers

I've swapped the DME relay when it failed to start and no change. Both distributors are good.

I can't find a schematic for the ignition module. Is it likely for both channels to fail together? I've seen posts where people have lost one of the coil outputs. I know that generically an ignition module could cause both of my problems, but I'm not clear how it functions with two coil outputs on the 1995 993.

systemsc

You need to start by checking for 12V at pins 18, 27, 37 with the key-on at the DME ECM. The two ignition signal outputs from the DME ECM are pins 25 & 31 to the spark module.
The ignition coils should have 12V with the key-on. The RPM signal input to the DME ECM is on pins 48 & 49. The engine will run with just one coil output.

pp000830

and a lot less expensive too.

Test the distributor by running the engine on each individually as discussed here.
https://993servicerepair.blogspot.co...ling-dual.html
Does it run better on one vs the other?

Also, have you checked that the wires lead to the correct plugs as discussed at the bottom of this page:
https://993servicerepair.blogspot.co...r-robbing.html

If the fuel filter is original to the car it may be clogged impeding fuel flow?

All stuff to test for.

Andy

drchambers

Thanks, I'll try this again but generally by the time I grab a voltmeter and get to the DME or module harness the car will decide to start again and of course everything tests fine. I need to wire in a test harness that I can get to quickly...