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Matt, Engineer
Category: Chevy
Satisfied Customers: 20391
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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Vehicle jerks, hesitates, and stutters. Really bad when in

Customer Question

Vehicle jerks, hesitates, and stutters. Really bad when in 5th gear and trying to just keep steady at 60mph. It doesn't bother when accelerating in 5th gear. I suspected the PMD, so I replaced it and it did not solve the problem. The PMD was bad about 2 months ago and caused the SES light to come on. Now I have very similar problems, with no lights or codes. It is a 6.5 diesel with a 5 speed manual trans. and 4.10 rear end with 175,000 miles. It has some excessive blowby, but I don't believe that is part of the problem.
Submitted: 2 months ago.
Category: Chevy
Expert:  Matt replied 2 months ago.


1st thing to check is if the engine is fitted with a MAF ( mass airflow sensor) as 1995 was the introduction year

the sensor is fitted just after the air filter box and forms part of the air inlet duct

if it does have a MAF then this could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading

Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

Customer: replied 1 month ago.
I don't believe they have a MAF sensor on the 95 6.5 turbo diesel. The only sensors on the intake are the air intake temp sensor, and the boost sensor, which are both mounted right on the intake on top of the engine. The engine also idles rough and surges up and down while idling.
Expert:  Matt replied 1 month ago.


if its the non MAF setup then it'll be the MAP sensor setup so its well worth removing the MAP / boost sensor from the manifold and checking its connections and that the sense port isn't blocked

Customer: replied 1 month ago.
I tried a new boost sensor on the intake, and it didn't seem to fix the problem. The connector tab is broken, but even if I push it in tight it doesn't fix the problem. I stepped on the pedal at idle, and it revved up to 2,000 rpm and stuck there. I had to shut it off to release it. Maybe that will give you another idea. Thanks, Jim
Expert:  Matt replied 1 month ago.

Hello Jim

OK as a next step then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.

Customer: replied 1 month ago.
Still working on checking the EGR, but here is what happened today. At idle, I lightly stepped on the pedal and it revved up to 2k rpm. It would not come back to idle. I lightly tapped the pedal and it revved up to 35-4000 rpm and I had to turn it off to get it to stop.
Expert:  Matt replied 1 month ago.


OK something else that worth checking then is if the oil level is dropping quicker than normal

as its possibly drawing in engine oil through the breather , or the turbo and then running off that?

The first thing I'd check is the crankcase pressure relief valve (PCV) these are sprung loaded and are only meant to 'burp' excess pressure every now and then. The exiting gas is piped back to the inlet, however should the valve stick open then the engine tends to suck the air out of the crankcase all the time and this carries oil over with it.

If you remove the air intake pipe and look inside where the crankcase breather is plumbed in you may find evidence of oil.

If you do find oil replace the valve

. If this is all OK then its possible that the valve stem seals are past their best and are drip-feeding oil into the intake. These can be replaced without removing the head by a competent garage in half a day or so

Also check inside the intake just after the turbo for any signs of oil as this would be an indication that the turbo oil seal is starting to fail and a replacement /rebuilt turbo may well be required. Have a feel of the turbine wheel - there should be no play radially at all - but a little bit of axial / end to end float is normal

an engine flush may also help if you've got a sticking oil scraper ring so its worth a go, asfor the oil if you go up a grade / use slightly thicker oil then this is less readily drawn into gaps and less likely to be burnt off

Expert:  Matt replied 1 month ago.


do you still need help?

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