Hi all, 84 760ti w/ 350k km. Canadian Eh. I have had this problem since buying the car. When driving, never at idle, at around 2400 to 2800 rpm the car will lurch or cut out. I have watched the tach and it doesn't drop, this is only a split second then it is back running normal, but it happens often. I have recently replaced O2 sensor, temp sensor, cleaned throttle body, replace major grounds and searched for vacuum leaks. Is this an easy fix or should I just keep replacing all vacuum lines and cleaning grounds? Thanks in advance.Grant.

Have you checked the engine wiring harness. Look at the insulation and if it's crumbling, it's time for a re-wire job, or a replacement. Resolder the fuel pump relay anyway. Look at the contacts in the socket, and the socket itself, for heat related melting or discoloration. The connectors may need to be replaced too. Ask me how I know.

I had some strange problems caused by cooked wires in the fuse and relay tray. Once I fixed the cooked wires, my problems went away.
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Warren Bain - '86 744Ti M46 >285Kmi, '96 965 >99Kmi Wifemobile near Manassas Va.. I get no e-mail replies to my postings.

Thanks for all the advice guys, looks like I have my work cut out for me this weekend. I will either replace or resolder the relay and check for any crumbling wires on the wire harness and inspect the fuse and relay conectors. Grant

Because the tach stays steady and you have taken care of the secondary ignition side I vote that you have a fuel issue and I further submit that it is the fuel pump relay. Sudden momentary blips in power is not a symptom of a clogging fuel filter or typical of a bad pump but is certainly the failure mode for the relay. Don't repace it just resolder it.

David:

So here is the question then: if I replaced my relay with a brand new one (not even resoldered... straight from the supplier) why would I still have the stutter when cruising at highway speeds?

It feels like a fuel issue, and specifically seems to me like its a small hiccup/pocket of air (very small) in the fuel system. However, I've replaced my pre-pump, fpr, and fuel relay, and Moons wrote he has replaced his filter and main pump, and neither set of solutions has fixed the problem.

Could this be tied to some other problem? Is it possible the relay (whether new or resoldered) heats up (as it is prone to) and cuts out momentarily?

Interested in any opinion you might have.

Thanks,
-Rick

Rick, your comment about an air pocket in the fuel system made me think of my gas tank. I knew when I bought the car that the tank has a slight leak at the top where it seals together, nothing major, it never leaves fuel on the ground but I know it is there. Is the tank supposed to be under pressure and would air entering through this leak cause any problems? As well I talked to my mechanic friend and he thinks it is vacuum related because it only happens at 2400-2800 rpms, not on acceleration or decceleration. It was a short conversation and I plan on getting more info before the weekend when I will be attempting a fix, I will post my results. Thanks, Grant.

I don't think a crack in the top of the gas tank or other hole in the tank would cause this problem. The gas tank is under pressure, but only at times, and then only slight pressure caused by the removal of gasoline as the car is driven. However, I think I'd probably replace that tank at some point in the near future... the idea of driving around with an already split tank is a bit petrifying (more likely to split in half in a collision if its already partly there).

The vacuum idea sounds valid, but the question is where?!? I've checked all the hoses in my vehicle repeatedly, and cut off and moved up the ends of ones that were going bad. I do have a slight vacuum loss that I can hear when I shut off the engine... it hisses slightly directly in front of the firewall on the driver's side. However, I think this is off the turbo line since the turbo gauge rides up to the top of the black when the car is off (resting position is bottom of black).

Crud, I don't have a clue. Let me know if you pin it down or think about where it might be happening.

-Rick

The sporadic miss felt only on the freeway in the wife's 86 745T was completely cured by re-soldering the Fuel Injection Relay.

JS:

So here is the question then: if I replaced my relay with a brand new one (not even resoldered... straight from the supplier) why would I still have the stutter when cruicing at highway speeds?

It feels like a fuel issue, and specifically seems to me like its a small hiccup/pocket of air (very small) in the fuel system. However, I've replaced my pre-pump, fpr, and fuel relay, and Moons wrote he has replaced his filter and main pump, and neither set of solutions has fixed the problem.

Could this be tied to some other problem? Is it possible the relay (whether new or resoldered) heats up (as it is prone to) and cuts out momentarily?

Interested in any opinion you might have.

Thanks,
-Rick

This is a known issue that no one (that I;ve heard of) has been able to resolve. I've posted on it and seen others post on it, but I've never come across an answer.

My analysis has led me to conclude it is related to the fuel system, not the various components you mention. To that end I have replaced my pre-pump, fuel relay (brand new, not resoldered), fuel pressure regulator, and cleaned out the lines + injectors. The two elements I have not replaced (but believe are central) are the main pump and fuel filter. I actually believe the latter of these two to be the central player, but I can't for the life of me get mine off; it appears to be slightly rusted on and was overtightened when last replaced. While I'm not a herculoid I'm a pretty strong guy, and all my strength with two vice grips couldn't get the damned thing to budge.

Anyway, it could be an electrical reset or it could be a little air or fuel hiccup causing it to stutter quickly. It happens so fact its hard for me to tell, although I never notice any lighting changes at night to indicate it is electrical. The fact that you replaced all those components and still didn't fix it reinforces my belief that it is a fuel issue.

Hope this helps. Make sure to post any follow-ups to this thread... I'd be interested in hearing anything you come up with.

Good luck,
Rick

Thanks for your insight rick, your intuition is similar to my own. I replaced the fuel pump and filter about 6 months ago, did not notice a difference and yes it is a bitch to get off. I also replaced the o rings and caps on my injectors. The next thing was going to be the fuel relay and the intank pump, now I am not too sure that will help. I know a great mechanic I am going to bug about this one, I will see what he says about it and post it. Thanks for the help, Grant.

The best term I and others on this board have found for this problem is "engine stutter". Never happens except when driving, and generally happens at highway speeds.

Very annoying to hear the filter/main replacement didn't fix it... argh. I have an independant volvo (read old model guy) specialist as well, so I'll have to pester him to see if he has ever resolved one of these. I will post to the bboard as well should I find anything.

Good luck,
Rick

I resoldered the fuel pump relay on Sat., No luck, stutter is still their at 2600 rpms. The good news is I finished my K&N air filter retro, looks sharp, sounds good and more HP. Grant.

Have you checked the spark plug leads and the dist. cap and rotor? my leads are the reason my car lurches, but at $150 aussie dollars for new leads, i love the lurching......
Az

Oops, I forgot too mension that, I just replaced the cap, rotor, leads, and plugs, all with bosch parts and that did not fix it either. It was expensive here in Canada, but not that expensive. I hate the lurch.