Breakaway944

'91 Cab with turbo engine swap running A - Tune, F-Tech DME relay w/prime. Finally on the road and have been debugging for a couple of weeks. Normally I go out and drive for 30 min to an hour and come back. I usually start the engine up within five minutes to make sure I do not have a no start problem. Always started right up. The problem is when I leave the car sitting for 20 minutes and try and start it, obviously no start. At this time when I turn the key on I can hear the fuel pump prime but when I crank the engine there is no tach bounce. The sensors are new Bosch and ohm out correctly from the plug. As the car cools down the first thing that happens is that when I turn the key to the on position (right before start) I get one or two tach bounces up to 500+-rpm, larger then the normal bounce. Then when I crank the engine no bounce. This happens about 10 - 15 minutes before it will start. About 5 minutes before start, when I turn key to the on position the tach will peg and the fuel pump goes on full - the car thinks the engine is doing huge rpms! This last time I replaced the FTech relay with the original one from the car. When the tach pegged the fuel pump did not run (bad relay) but heard a crackling sound. Traced it to the distributor cap, sparks going off. I did not hear this with the FTech relay because the fuel pump was too loud.
Any ideas what could short and make the DME think the engine is doing 8k+ rpms? I replaced the connectors for the relays on the harness side with no change. I have ordered the replacement harness for the speed/ref and waiting for it to come in.

GPA951s

A hot start issue is typically related to residual fuel pressure. The check valve is bad and the air is entering the system.. without the pressure in the system you are getting vaporlock. Check your residual fuel pressure…

CyCloNe!

I had a hot running issue that was the speed/ref sensors. If I was on the interstate it would stay running but would die if I slowed for traffic/stopped. If it idled till warm it would die within 10min. Then it wouldn't start until it cooled. Your sensors are testing fine so prob not them but doesn't hurt to check the harness and make sure the magnet ends of the sensors are clean.

Breakaway944

I am 99% sure it is an electrical issue. The only thing that is not new/rebuilt in the fuel system is the rail, damper and hard lines. New Bosch pump and 3 bar fpr, all lines/filters replaced, rebuilt injectors and LR injector harness. I did reroute the lines in the engine compartment to loop around to the firewall and come forward instead of going over the headers and splitting right/left to help avoid vapor lock. I can't imagine vapor lock causing the DME to think the car is at 7000+ rpm.
I will be doing more testing today. I bought a used harness way back when and took those wires for the sensors and put them into my harness since they looked better. I will recheck that wiring. I had the power probe out when I was checking the resistance on the sensors and decided to see what readings I would get with the three wires on the harness side of the connector. The ground on both connectors checked out perfect. On one connector both of the other wires showed a .7v pulse - half second on and half second off. The other connector the outside wire seemed to have the same pulse and it seemed like the middle wire would ground now and then. This is with no key, car off. Car starts fine like this.
One more strange thing is that both my dmm and power probe are showing battery voltage at 11.8v. The battery is less then one month old. At this voltage the car should not even crank, but it cranks fast and strong like it is at 12.7. Is there something drawing a volt from the system?
Other wiring I will look at this weekend is stereo wiring. My amp takes 15+- seconds to turn off. Could this be related? Could I have used a power antenna lead for an amp switch and the lead stays energized for a bit to put antenna down?
Did these cars come with RAP (retained accessory power)? I was surprised to learn my windows still work for a while after car turned off. Did not think RAP was common back then. Should read the owners manual I guess!

Breakaway944

At this point I think I can rule out dme relay and speed/ref sensors. The sensors always ohm out 900 cold and 1200 hot. Tested at plug and dme connector. The strange readings I am getting are the pulsed .7v on the signal wires as tested at sensor plug in engine compartment. When I unplugged the dme I could have sworn I got a steady ground on pin 26, then a sporadic ground (I had just put the car into a heat cycle to trigger the tach pegging, turned car off, unplugged sensors, unplugged dme and tested). I thought for sure it was the reference sensor wire. In the last post I mentioned sporadic ground on the middle pin of the ref sensor when tested from engine compartment. I had an old sensor wire from the original harness and swapped it out. I had to splice in the connector I used from another sensor onto it. This did not solve my problem. Same symptoms. I have a new harness arriving Friday to rule this out.
With the tach pegged I checked my AVC-R and it was reading 9000-10000 rpms. Since the dme was lying on the floor I gave it a bump and the rpms went away. When the tach pegs the fuel pump runs, the coil sparks like crazy but the injectors do not fire - thankfully.
A possibility is that the ref sensor plug is shorting. Seems slim because tapping the dme cures the problem and the tapping does not transmit to the plug in the engine compartment.
Could be loose connection at dme. Maybe the new harness will correct it? Perhaps my ground is not that great?
Could be speed sensor wiring. Again, will find out this weekend with new harness.
Could be bad solder joints in dme or bad drivers. What triggers the dme to see 10k rpms? I thought it was the ref sensor? If it is not that what else could make the dme think the car is running 10k?
The other possibility is a low battery causing weird stuff to happen. Every time I test the battery it is 11.8-12v. 12v is only 25-40% charged. When I first started testing the car the batt gauge would read 12.2 car running. The battery would always test at 12.7 car off, and 13.7 car on. Now the gauge reads 13+ when running. Have not tested it recently while running to verify I am still getting 13.7. The battery is definitely charging, but maybe not enough to fully charge batt? Will test running voltage tomorrow to verify alt working. Put the battery on charger to fully charge it tonight.
Any thoughts on what it could be would be appreciated!

Dan Martinic

With an engine swap, who knows what has happened to the wiring and grounding systems. I would imagine there are some differences between '91 and earlier cars' wiring harnesses etc. In any case, since you mention tapping the ECU/DME seems to alter the behaviour, it would not hurt to check for cracks in the solder joints.

You can barely see them. I used the phone camera on zoom to find these "lines" then re-soldered. Fixed some weird issues I was having. There were lots of joints with these..

Breakaway944

I am thinking you are right but there is one more slight possibility I can think of. I have not done much with the car in the past year. I fixed a couple of little things. One thing I did was replaced the microswitch in the drivers handle, I could not turn the alarm on/off before. Now it worked. A couple of months ago the alarm went off for no reason. I turned it off from the drivers door lock and did not think much about it. Since then I have not locked the door, do not need the alarm going off randomly. Two days ago I came out and the drivers door was locked. I tested the lock system and the drivers door lock is not working with the key or lock button. I remember reading a "no start" thread that was traced down to the alarm and drivers door lock actuator. This was in a '91 with the new alarm system like mine. When I tapped the DME to make the car rev down from 10k rpm's I contributed it to the DME and or wiring right there. Looking at it again the alarm module was sitting right on top of the DME and therefore would have been shaken too.
I think you are probably 99% correct (what would the alarm have to do with rpm?), but if not I will look at alarm. Anyone know how to disable the '91+ alarm system?
My current soldering iron is a pos so will buy new one today and go through the solder joints - on both dme and alarm.

Breakaway944

FTW
Took apart DME and went for the coil driver. Re flowed the three points and put it back together. The box had been repaired before at some point and a lot of points had new solder on them, but not the driver. Put in car last night and drove it 30 minutes. After shut off I tried starting it every 10 minutes for an hour and it always started right up.Today drove it 180+-miles, with half a dozen stops of varying time, and it worked perfect. So it should work as long as my crappy soldering job holds!

Dan Martinic

I hope this solves your issues.. if not, at least you've ruled another thing out