Engine cut out / stall on highway

[Techno Duck]

I have a few things to check on my '88 3.2L but appreciate any other input.

While on the highway heading home, cruising at about 70mph (just a bit over 3k rpm), while going over some minor bumps / dips in the pavement the engine cut out briefly. Tach dropped like the engine shut off and i felt the loss of power. Lasted only a brief moment, then power came right back and continued cruising no problem. Further on down the road while changing lanes, as soon as the front wheel hit the group of reflectors dividing the lanes the engine cut out completely and stalled. I coasted onto the shoulder to a stop and restarted. Engine cranked over and started back up without much trouble. The cut out of the bumps happened a few other times and it stalled a total of two times during the rest of the trip home. All of this occurred while going over dips or with excessive vibration from the road.

Interestingly enough i experienced the cutout briefly on this same exact stretch of road a few months back during the last 200 mile round trip i made. I could not get it to replicate and did not think much else of it.

Other details;
-70 degree ambient
-Dry conditions; car made it all the way to LA in the morning with wet roads, but i am certain the car was totally dry at this point.
-3/4 tank of gas
-All vacuum hoses, intake gaskets..etc done a few months back along with speed / ref sensors. Car runs great otherwise.
-CHT sensor is the 2-wire type; i did not replace it.
-Fuel filter is less than a year old.
-Injectors are rebuilt from 3rd party supplier
-New fuel pressure regulator
-DME relay is new
-Cap and rotor have less than 3-4k miles on them.
-Ignition wires have 10k miles
-Most of the grounds on the car have been cleaned over the past year, all battery connections clean and tight also

Here is what i was thinking...

1) To me it almost felt like a fuel cut is happening which makes me suspect the fuel pump. The pump is quiet, but perhaps during extended running (1.5+ hours) on the highway its getting weak. The pump is original as far as i know with 165k miles on it.

Unfortunately i filled up after the first time it stalled, so i have like 20 gallons in the tank to siphon out. The plan is to replace the pump, tank screen and the pump suction hose which i am sure is dry rotted. I wanted to do this anyhow eventually so i figured a good place to start.

2) DME solder joints; perhaps bad solder joints in the DME, after driving an hour or so the DME builds up enough heat combined with road vibration to cause an issue. I plan to send the DME somewhere for testing / inspection aswell for peace of mind. I would be more inclined to think though if it were a DME issue than the car would just flat out stall rather than feel like it cuts out? I plan to reinstall the DME on some rubber mounts to help absorb vibration; maybe some bubble wrap . Ill pull the DME out when i get a chance and give it a once over for cracked joints.

3) Ignition switch is original i believe also. Perhaps the contacts are getting weak? I dont think this is it because i would suspect the problem to happen all the time. Not just after an hour or so of driving.

[patz]

Bad fuse that has worn contacts for power to DME or a DME relay ready to crap the bed. Check the fuse and swap out the relay. Anything NOT made by URO.

[timmy2]

Check the Red fuel pump relay in the front fuse panel. It may need the pins spread out with a razor blade. It could be a little loose in the socket, and when you hit a bump it lost contact.
The clue is the front wheels on the bumps as a causal factor.

[jmz]

Check your ground wires too. ...battery and under transaxle. At least that's caused problems similar for me on my older 911.


sorry just read that you've checked grounds.

[Rodsrsr]

Originally Posted by patz

Bad fuse that has worn contacts for power to DME or a DME relay ready to crap the bed. Check the fuse and swap out the relay. Anything NOT made by URO.

I second this ^^^^^^^ dirty fuse or DME relay. there is no red relay on the 88 btw.

[Hoss911sc]

it could be the ignition switch - happened to me with intermittent issues. How much play with key in ignition? When I got my new one suddenly it felt very tight. It happens gradually over time so it's not so noticeable. Also check that the connector on back of ignition switch is tight then follow the wiring to the plug connector on the firewall (white). That can get loose too and cause power issues and fry wiring (happened to me).

Good luck, hope this helps.

[T77911S]

check the battery connections. clean

if you can solder pull the DME relay and hit all the joints with an iron.

rotate all the fuses, it cleans the contacts

[jpnovak]

Does it do the same in 4th gear with higher rpm? Could be worn carbon tracts on your AFM sweeper arm.

[cabmandone]

I know you mention newer DME relay but that doesn't really mean that much from what I've read. You might want to switch with a known good one to rule it out. If it loses contact even briefly it will kill the fuel pump. You also mention that the ground have been cleaned recently but I think on an intermittent problem such as yours I'd go back and check that the nut is tight for the battery ground, Transmission to chassis ground and the intake runner ground. I'd also do a visual of all the plugs in the engine compartment to make sure they are making solid contact and that one isn't missing one of the clips that hold them on.

[jlex]

Try tapping on that DME relay while the engine is running & see if it stumbles.

[DRACO A5OG]

Check the Dizzy.

If there is side to side & up and down slop, remove it and confirm. There should be 0.5MM play up and down but there should be no side slop.

[rekstein1]

Having an 88 as well and recently going through a no start issue. I would check out sugarwoods method of testing the DME relay that could save you $35 bucks. Its a 9v battery and 1.99 led from radioshack and some clips for connections.

Second I would jump the fuel pump at the fuse panel and hear the fuel pump whirls its fuse 6 I believe. it in the article. Before opening up the DME i would look into the pelican article about no start issues. I would test the sensors of the CHT, and the others at the pins. Key items to a engine running smoothly

If everything checks out then I would look to the DME. Ask me how I know

[ischmitz]

Ignition switch, DME relay, FW sensors, DME come to mind.

Easy decision on the DME relay: get a new one as you should have a spare anyway.
The ignition switch (electrical part) is not that cheap so testing would be what I'd do.
The sensors can be intermittent and at 80$ per it's not a bad idea to replace them after almost 30 years.

If the tach dropped this would point towards power or sensors rather than DME itself. Start getting out the wooden screwdriver handle and tap things while the engine idles.

Ingo

[cabmandone]

Originally Posted by ischmitz

Start getting out the wooden screwdriver handle and tap things while the engine idles.

I was going to suggest that method of troubleshooting too but the technical side of me said NO don't do it.

[ischmitz]

It's either that or throw parts at it. For the DME relay it's fine. For the DME tap it gently with you flat hand, ideally when it's not mounted to the floor.

[cabmandone]

Originally Posted by ischmitz

It's either that or throw parts at it. For the DME relay it's fine. For the DME tap it gently with you flat hand, ideally when it's not mounted to the floor.

Quick question since I know you are VERY well versed in this, if he lost tach why would it possibly be the speed and reference sensors? It's my understanding that they have nothing to do with tach signal. Wouldn't it point more to a loss of power at the DME either from fuse power or internally?

[ischmitz]

Once the speed sensor goes away the DME heartbeat (fuel signals, spark signals, tach signals) from the DME go away. And the fuel pump stops. The reference sensor is actually less suspect here. It is only needed once at startup but then only get read sporadically.

A lost speed sensor signal or lost power will look identical unless you are able to see what the ICV does when the system cuts out. You really need a data logger + breakout box to accurately diagnose this.

[captain hook]

Had similar problem... had a zillion theories. . Turned out to be a worn coil wire.

[4flyboy]

Yep, same here, simple as a coil wire. Weird too, it looked fresh on the outside...

[Curt K]

Something similar happened to me with my 87 - I went through a puddle at speed and could feel the water hit the floor boards. Engine cut out, but I was able to restart it. A couple days later, the car would not start. Short version - after several weeks of trouble shooting, it turned out to be cracked solder joints in the DME. Based on information I found in other threads, I opened the DME and examined the board, re-soldered the faulty joints, and have been fine for many years.

My guess would be that going over the bumps opened the cracks, shutting down the DME signal. Just a suggestion to look over the DEM with a good magnifier. Another thing to eliminate as a culprit.